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من طرف Basic immunology
The immune system is the body’s natural defence in combating organisms.
Immunology has developed rapidly over the past decade owing to the refinements made
in the molecular tests employed in this area of research. Therefore, the keen reader is
encouraged to peruse the ophthalmic and immunological literature in order to keep
abreast of the latest developments in this field.
The College of
Optometrists has
awarded this article 2
CET credits. There are
12 MCQs with a pass
mark of 60%.
Owing to the complex nature of this subject, it is
far beyond the scope of this article to cover all
aspects of immunology. Rather, the aims of the
article are twofold: first to acquaint the busy
practitioner with the basic concepts of the
immune system; and second, to introduce the
reader to the more specific topic of ocular
immunology – the study of the ocular immune
system.
Finally, since it is envisaged that optometrists
will one day prescribe therapeutic agents, the
discussion is limited to the anterior segment and
anterior uvea.
Innate & adaptive immune systems
The immune system can be thought of as having
two “lines of defence”: the first, representing a
non-specific (no memory) response to antigen
(substance to which the body regards as foreign
or potentially harmful) known as the innate
immune system; and the second, the adaptive
immune system, which displays a high degree of
memory and specificity. The innate system
represents the first line of defence to an
intruding pathogen. The response evolved is
therefore rapid, and is unable to “memorise” the
same said pathogen should the body be exposed
to it in the future. Although the cells and
molecules of the adaptive system possess slower
temporal dynamics, they possess a high degree
of specificity and evoke a more potent response
on secondary exposure to the pathogen.
The adaptive immune system frequently
incorporates cells and molecules of the innate
system in its fight against harmful pathogens.
For example, complement (molecules of the
innate system - see later) may be activated by
antibodies (molecules of the adaptive system)
thus providing a useful addition to the adaptive
system’s armamentaria.
A comparison of the two systems can be seen
in Table 1.
ot
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Gregory Heath BSc (Hons), MCOptom, Dip. Clin. Optom
ABDO has awarded this
article
2 CET credits (GD).
26 February 8, 2002 OT
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Cells of the innate
immune system
Phagocytes
Although sub-divided into two main types,
namely neutrophils and macrophages, they
both share the same function - to engulf
microbes (phago - I eat, Latin).
Neutrophils
Microscopically, these cells possess a
characteristic, salient feature - a
multilobular nucleus (Figure 2). As such,
these cells have been referred to as
polymorphonuclear leukocytes (PMNs) and
have a pivotal role to play in the
development of acute inflammation. In
addition to being phagocytic, neutrophils
contain granules and can also be classed as
one of the granulocytes. The granules
contain acidic and alkaline phosphatases,
defensins and peroxidase - all of which
represent the requisite molecules required for
successful elimination of the unwanted
microbe(s).
Macrophages
Macrophages (termed monocytes when in the
blood stream) have a horseshoe-shaped nucleus
and are large cells. Properties of macrophages
include phagocytosis and antigen presentation
to T cells (see later). Unlike neutrophils (which
are short-lived cells), they are seen in chronic
inflammation as they are long-lived cells.
Mononuclear phagocytic system
The cells comprising the monocyte phagocytic
system are tissue bound and, as a result, are
further sub-divided depending on their location.
A list of the cells together with their
corresponding location can be found in Table 2.
Table 1: Cells and molecules of the innate and adaptive immune systems
Immunity Cells Molecules
Innate Natural killer (NK) cells Cytokines
Mast cells Complement
Dendritic cells Acute phase proteins
Phagocytes
Adaptive T and B cells Cytokines
Antibodies
Components of the immune system can be seen in Figure 1.
Figure 1 The principle components of the immune system are listed, indicating which cells
produce which soluble mediators. Complement is made primarily by the liver, with some
synthesised by mononuclear phagocytes. Note that each cell only produces a particular set of
cytokines, mediators etc
Figure 2 Morphology of the neutrophil. This
shows a neutrophil with its characteristic
multilobed nucleus and neutrophilic granules
in the cytoplasm. Giemsa stain, x 1500
www.optometry.co.uk 27
Table 2: Examples of cells of the
mononuclear phagocytic system and their
respective locations
Cells Location
Monocytes Blood stream
Alveolar macrophages Lungs
Sinus macrophages Lymph nodes
and spleen
Kupffer cells Liver
Dendritic cells
Dendritic cells consist of Langerhans’ and
interdigitating cells and form an important
bridge between innate and adaptive immunity,
as the cells present the antigenic peptide to the
T helper cell (adaptive immunity). Such cells are
therefore known as professional antigen
presenting cells (APCs). Table 3 illustrates the
various types of dendritic cells together with an
example of their location.
Eosinophils
Eosinophils (so called because their granules
stain with eosin – Figure 4) are granulocytes
that possess phagocytic properties. Despite the
fact that they represent only 2-5 % of the total
leukocyte population, they are instrumental in
the fight against parasites that are too big to be
phagocytosed.
Phagocytosis - the process
Phagocytosis is the process by which cells engulf
microorganisms and particles (Figure 3). Firstly,
the phagocyte must move towards the microbe
under the influence of chemotactic signals, e.g.
complement (see later). For the process to
continue, the phagocyte must attach to the
microbe either by recognition of the microbial
sugar residues (e.g. mannose) on its surface or
complement/antibody, which is bound to the
pathogen. Following attachment, the
phagocyte’s cell surface invaginates and the
microbe becomes internalised into a
phagosome. The resultant phagosome fuses with
multiple vesicles containing O2 free radicals and
other toxic proteins known as lysosomes to form
a phagolysosome. The microbe is subsequently
destroyed.
Opsonisation (“to make tasty” - Greek)
Opsonins are molecules, which enhance the
efficiency of the phagocytic process by coating
the microbe and effectively marking them for
their destruction. Important opsonins are the
complement component C3b and antibodies.
Natural killer (NK) cells
NK cells are also known as “large granular
lymphocytes” (LGLs) and are mainly found in the
circulation. They comprise between 5-11% of the
total lymphocyte fraction. In addition to
possessing receptors for immunoglobulin type G
(IgG), they contain two unique cell surface
receptors known as killer activation receptor and
killer inhibition receptor. Activation of the
former initiates cytokine (“communication”)
molecules from the cell whilst activation of the
latter inhibits the aforesaid action.
NK cells serve an important role in attacking
virally-infected cells in addition to certain
tumour cells. Destruction of infected cells is
achieved through the release of perforins and
granyzymes from its granules, which induce
apoptosis (programmed cell death). NK cells are
also able to secrete interferon-γ (IFN-γ). This
interferon serves two purposes: first, to prevent
healthy host cells from becoming infected by a
virus; and second, to augment the T cell
response to other virally infected cells
(see later).
Mast cells and basophils
Morphologically, mast cells and basophils are
very similar in that both contain electron dense
granules in the cytoplasm. Basophils are
so-called owing to the fact that their granules
stain with a basic dye. Unlike mast cells, which
are present in close proximity to blood vessels in
connective tissue, basophils reside in the
circulation.
Both cell types are instrumental in initiating
the acute inflammatory response. Degranulation
is achieved either by binding to components of
the complement system or by cross-linking of
the IgE antibody which results in the release of
pro-inflammatory mediators including histamine
and various cytokines. The former induces
vasodilation and augments vascular permeability
whilst the latter are important in attracting
both neutrophils and eosinophils.
Table 3: Dendric cells and location
Cells Location
Langerhans cell Limbus, skin
Interdigitating cell T cell areas in
lymph nodes
Figure 4 Morphology of the eosinophil. The
multilobed nucleus is stained blue and the
cytoplasmic granules are stained red.
Leishman stain, x 1800
Figure 3
Phagocytes arrive at a site of inflammation
by chemotaxis. They may then attach to
microorganisms via their non-specific cell
surface receptors. Alternatively, if the
organism is opsonised with a fragment of
the third complement component (C3b),
attachment will be through the phagocyte’s
receptors for C3b. If the phagocyte
membrane now becomes activated by the
infectious agent, it is taken into a
phagosome by pseudopodia extending
around it. Once inside, lysosomes fuse with
the phagosome to form a phagolysosome
and the infectious agent is killed. Undigested
microbial products may be released to the
outside
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February 8, 2002 OT www.optometry.co.uk
for the successful eradication of an invading
virus by the innate immune system.
Type II IFN, IFN-γ, is produced by T Helper
cells and NK cells and is able to augment both
the antigen presenting properties together with
the phagocytic properties of the APCs (e.g.
macrophages and dentritic cells).
Adaptive immunity
As mentioned previously, there is a great deal of
synergy between the adaptive immune system
and its innate counterpart. The adaptive immune
system comprises two main types of leukocyte
known as B and T lymphocytes. Before describing
these important cell types, it is necessary to
acquaint the reader with both the primary and
secondary lymphoid organs and tissues in the
body. These are summarised in Table 4.
The bone marrow represents the dominant site
for haemopoiesis (production of blood cells and
platelets). Although most of the haemopoietic
cells maturate in this region, T lymphocytes do
so in the thymus. In the thymus, premature T
cells undergo a process of positive and negative
selection whereby the former are allowed to
progress to maturity whilst the latter are marked
for termination via apoptosis (see central
tolerance).
Lymphocytes
Morphologically, there are three types of
lymphocytes: T, B and NK cells. However, only T
and B lymphocytes exhibit memory and
specificity and, as such, are responsible for the
unique quality of the adaptive immune system.
Resting B lymphocytes are able to react with
free antigen directly when it binds to their cell
surface immunoglobins which act as receptors. T
lymphocytes do not react with free antigen and
instead make use of APCs to phagocytose the
antigen and then to express its component
28
Molecules of the innate
immune system
There are many molecules, which work in concert
with the cells of the innate immune system and
which also foster close functional links with their
adaptive counterpart. The three major molecules
are:
• Complement
• Acute phase proteins (APP)
• Interferons (IFNs)
Complement
The complement system represents a large group
of independent proteins (denoted by the letter C
and followed by a number), secreted by both
hepatocytes (liver cells) and monocytes.
Although these proteins maybe activated by both
the adaptive immune system (classical pathway)
or innate immune system (alternative pathway),
the nomenclature is derived from the fact that
the proteins help (“complement”) the antibody
response.
Activation of complement via the microbe
itself is known as the alternative pathway. The
classical pathway requires the interaction of
antibody with specific antigen. The C3
component is the pivotal serum protein of the
complement system. Binding of the antigen to
C3 results in two possible sequelae. In either
case, C3 component becomes enzymatically
converted to C3b. The bacterial cell wall can
either remain bound to C3b and become
opsonised (since phagocytes have receptors for
C3b) or act as a focus for other complement
proteins (namely C5, 6, 7, 8 and 9). The latter
form the membrane attack complex (MAC), which
induces cellular lysis.
The functions of the complement system may
be summarised as follows:
• Opsonisation
• Lysis (destruction of cells through damage/
rupture of plasma membrane)
• Chemotaxis (directed migration of immune
cells)
• Initiation of active inflammation via direct
activation of mast cells
It is important that complement is regulated to
protect host cells from damage and/or their total
destruction. This is achieved by a series of
regulatory proteins, which are expressed on the
host cells themselves.
Acute phase proteins
These serum proteins are synthesised by
hepatocytes and are produced in high numbers
in response to cytokines released from
macrophages.
Interferons (IFNs)
IFNs are a group of molecules, which limit the
spread of viral infections (Figure 5). There are
two categories of IFNs, namely type I and type II.
Type I IFNs maybe sub-divided further into IFN-α
and β. IFN-γ is the sole type II interferon. Type I
IFNs are induced by viruses, pro-inflammatory
cytokines and endotoxins from gram negative
bacterial cell walls. Their presence remains vital
proteins on the cell surface adjacent to special
host proteins called major histocompatibility
complex (MHC) class II molecules. As discussed,
antigen presenting cells which express MHC class
II molecules include dendritic cells and
macrophages. This “afferent” phase must occur
in order for the T cell to recognise the antigen.
The “efferent” phase occurs when activated
lymphocytes enter the tissue and meet antigen
again. This results in multiplication and secretion
of cytokines or immunoglobins in order to
destroy the antigen.
T cells
T cells can be broadly divided into both T helper
(TH) and cytotoxic T cells (Tc). Furthermore, TH
cells may be sub-divided into TH1 and TH2. The
former are pro-inflammatory T cells and
stimulate macrophages whilst the latter
orchestrate B cell differentiation and maturation
and hence are involved in the production of
humoral immunity (antibody mediated). T cells
express cell surface proteins, described by cluster
determination (CD) numbers. TH cells express CD4
molecules on their cell surface, which enable the
lymphocyte to bind to a MHC class II molecule.
The T cell receptor is unique in that it is only
able to identify antigen when it is associated
with a MHC molecule on the surface of the cell.
Cytotoxic T cells are primarily involved in the
destruction of infected cells, notably viruses.
Unlike TH cells, cytotoxic cells possess CD8 cell
surface markers, which bind to antigenic
peptides expressed on MHC class I molecules.
B cells and antibodies
(immunoglobulins - Ig)
B cells are lymphocytes that produce antibodies
(immunoglobulins) and can recognise free
antigen directly. They are produced in the bone
marrow and migrate to secondary lymphoid
organs. B cells are responsible for the
Figure 5
When host cells become infected
by virus, they may produce
interferon. Different cell types
produce interferon-α (IFN-α ) or
interferon-β (IFN-β); interferon-γ
(IFN-γ) is produced by some types
of lymphocyte (TH) after activation
by antigen. Interferons act on other
host cells to induce a state of
resistance to viral infection. IFN-γ
has many other effects as well
Table 4:
Primary and secondary lymphoid organs
Primary lymphoid organs Secondary lymphoid organs
Bone marrow Lymph nodes
MALT - mucosa associated lymphoid
tissue (includes bronchus, gut, nasal and
conjunctival associated mucosal tissues)
Spleen
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Module 4 Part 2
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development of antibody mediated immunity
known as humoral mediated immunity.
When activated by foreign antigen, B cells
undergo proliferation and mature into antibody
secreting plasma cells. The latter are rich in
organelles such as rough endoplasmic reticulum
and mitochondria, which confer their ability to
secrete soluble proteins (antibodies). Not all
proliferating B cells develop into plasma cells.
Indeed, a significant proportion remain as
memory B cells through a process known as
clonal selection. This process is vital in
eliminating the antigen should the body become
re-exposed to it in the future. T cells are also
clonally selected and this confers to the
production of T memory cells.
Although T and B cells behave differently,
both are able to recirculate around the body
migrating from blood to tissue and vice versa.
The ability to recirculate obviously increases the
efficiency with which cells of the immune system
can home onto the invading antigen.
Antibodies
Antibodies have two roles to play - the first is to
bind antigen and the second is to interact with
host tissues and effector systems in order to
ensure removal of the antigen
(Figure 6).
There are five different types (known as
isotypes) of antibody in the human immune
system - namely IgM, IgG, IgA, IgE and IgD. In
addition, there are four sub classes of IgG
(IgG1-4). The basic antibody unit consists of a
glycosylated protein consisting of two heavy and
two light, polypeptide chains. The region which
binds to the antigen is known as the Fab region,
while the constant region, Fc, not only
determines the isotype but is the region
responsible for evoking effector systems, e.g.
mast cell activation. The term immune complex
refers to the combination of antigen and
antibody and will be discussed later in the article
(see type III hypersensitivity).
The antibody isotypes together with their
corresponding function are illustrated in Table 5.
MHC
Major histocompatability complex (MHC) are cell
surface proteins classified as class I (also termed
human leucocytic antigen [HLA] A, B and C),
found on all nucleated cells and class II (termed
HLA, DP, DQ and DR), found on all antigen
presenting cells (APCs). MHC molecules are the
sine qua non of T cell induced immunity.
Clinically, there is a strong association
between HLA and certain systemic and ocular
diseases (see later).
Cytokines
Cytokines (also termed interleukins [IL] meaning
“between white blood cells”) are small molecules
that act as a signal between cells and have a
variety of roles including chemotaxis, cellular
growth and cytotoxicity. Owing to their ability to
control immune activity, they have been
described as the “hormones” of the immune
system.
Characteristics
Crosses placenta thus providing newborn with useful humoral immunity
High affinity
Predominant antibody in blood and tissue fluid
Large pentameric structure in circulation
Present in monometric form on B cell surface
Secreted form is predominant antibody in early immune
response against antigen
Reaches 75% of adult levels at 12 months of age
Exists in both a monometric and dimeric form
Secretory IgA (dimeric form) represents 1st line of defence against
microbes invading the mucosal surface, e.g. tears
Low levels in circulation
Increased levels in worm infections
Fc region has high affinity for mast cell thus involved in allergy
Antigen receptor on B cells
Absent from memory cells
Table 5:
Antibody isotypes and corresponding functions
Antibody
IgG
IgM
IgA
IgE
IgD
Table 6:
Various cytokines, their sources and functions
Cytokine Source Function
IL-1 Macrophages 1. T, B cell activation
2. Mobilisation of PMNs
3. Induction of acute phase proteins
IL-2 T cells Proliferation of T and NK cells
Il-4 Th2 cells B cell activation
Mast cells IgE response
IL-8 Macrophages Chemotaxis of PMNs
T cells
Fibroblasts
Keratinocytes
IL-10 TH2 cells B cell activation
Macrophages Suppress macrophages
IL-12 B cells Stimulate TH1
Inhibit TH2
TGF β (transforming growth factor) T cells Inhibits other cytokines
TNF α (tumour necrosis factor) Macrophages Inflammation
Figure 6
When a microorganism lacks the
inherent ability to activate
complement or bind to
phagocytes, the body provides
antibodies as flexible adaptor
molecules. The body can make
several million different antibodies
able to recognise a wide variety
of infectious agents. Thus the
antibody illustrated binds microbe
1, but not microbe 2, by its
‘antigen-binding protein’ (Fab).
The ‘Fc portion’ may activate
complement or bind to Fc
receptors on host cells,
particularly phagocytes.
Table 6 summarises some of the cytokines
pertinent to ocular immunology, their functions
and their progenitors. Since interferons have
been discussed earlier in the article, they have
been omitted from the table.
Central and peripheral
tolerance: nature’s way of
containing the immune
response
Since various cells of the immune system are
capable of reacting with self-antigens, it is
therefore essential that the human body has
mechanisms to suppress/eliminate autoreactive
cells. Failure to do so, can, in some cases, lead
to the development of autoimmune diseases
(see later).
Central tolerance
Central tolerance refers to the process whereby
both immature B and T cell lymphocytes, which
react against normal, healthy cells
(self-antigens), are eliminated via apoptosis.
Peripheral tolerance
This involves the removal of mature lymphocytes,
which are not tolerant to healthy cells.
Ocular immune privilege
There are numerous sites in the body whereby
tissue may be grafted with minimal risk of
rejection. Such regions include, inter alia, the
testis, thyroid lens, anterior chamber, cornea,
iris and ciliary body1,2.
It is important that immune privilege is not
simple interpreted as the host’s inability to
initiate an immune response to a transplanted
tissue. Rather, it is an area of the body in which
there exists a paucity of various elements of the
human immune system in response to an
antigen.
Factors
The factors purported by investigators that
contribute to the phenomenon of ocular immune
privilege include:
• Isolation from a vascular supply
• Isolation from a lymphatic supply
• Presence of a vascular barrier
• Ability to suppress the immune response
• Anterior chamber associated immune
deviation (ACAID)
Vascular supply
The healthy cornea is a good example of an
ocular site devoid of a vascular network. The
evidence to support the role a vascular network
has to play in the mechanism of graft rejection
is unequivocal since the risk of failure correlates
positively with the degree of host
vascularisation3.
Vascular barrier
There is a plethora of evidence in the
ophthalmic literature to support the existence of
a blood-ocular barrier. Furthermore, the same
said barrier encompasses different elements
including tight junctions between retinal
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30 February 8, 2002 OT
endothelial cells and the presence of junctional
complexes linking retinal pigment epithelial
cells4.
Lymphatic role
The fact that skin allografts were not rejected
following lymph node removal5 led investigators
to hypothesise that immune privilege was solely
due to the absence of the same said system at a
particular anatomical site. However, although
certain immune privileged sites do indeed lack
lymphatic drainage, others such as the testes6
and eye7 do possess such a system. It appears
that a proportion of the aqueous humour drains
via the uveoscleral pathway into the lymphatic
vessels in the head and neck.
The eye, APCs & MHCs
As mentioned previously, APCs, through their
ability to express MHC class II molecules, are
potent progenitors of the immune response.
Moreover, such cells are capable of activating T
cells within the tissue itself. It is therefore not
unreasonable to assume that a paucity of APCs
may play an important role in immune
privilege. In addition, failure to express MHC
class I molecule would make a tissue immune
against the lytic action of the cytotoxic T cells.
Although the aforementioned mechanisms
are theoretically plausible, cells expressing both
MHC class I and II molecules have been
detected in the eye. Table 7 illustrates the
relationship between histocompatability class
and ocular cell type.
It is noteworthy that the epithelial cells of
the crystalline lens are devoid of class I
expression14 and that the Langerhans’ cells
(class II expression) are absent from the central
cornea15.
It is interesting that not all cells, which
express MHC class II act as professional APCs in
the eye. Indeed, it has been shown that such
cells reside in the iris and ciliary body and not
only fail to present alloantigens to T cells, but
have the ability to suppress mixed lymphocyte
reactions16.
The failure to incite the inflammatory
response has attracted a great deal of interest
amongst ophthalmologists and immunologists
alike. It appears such suppression is achieved by
various factors present in the aqueous humour
(e.g. transforming growth factor - β).
Anterior chamber associated immune
deviation (ACAID)
As a result of experiments with rats,
investigators discovered that antigens placed in
the anterior chamber resulted in systemic
inhibition of delayed type hypersensitivity (DTH
or type IV hypersensitivity) reactions to the
same said antigens17. This phenomenon has been
coined anterior chamber associated immune
deviation (ACAID). The anterior chamber is thus
able to suppress delayed type hypersensitivity
reactions and inhibit the production of
complement fixing antibodies18,19. However, it
has no inhibitory effect on cytotoxic T cell
activity and has a minimal influence on the
production of non-complement fixing
antibodies.
With respect to the endothelium, two
adaptations prevent it from immunological
injury: first, avoidance of cytotoxic T lymphocyte
(CTL)-mediated lysis; and second, inhibition of
DTH responses in the anterior chamber22. It
achieves the former through the cells inability
to express MHC class I molecules21. The corollary
of this, however, is that virally infected cells
may persist in this region. The râison d’etre of
ACAID is to protect the eye from the DTH
response to pernicious antigens. As a result of
its location in relation to the anterior chamber,
the corneal endothelium seems well placed to
reap the benefits of ACAID.
ACAID is beneficial in reducing the incidence
of stromal keratitis in herpes simplex virus
infection. It therefore seems reasonable to
assume that such an unwanted corneal sideeffect
occurs as a result of a DTH response
rather than the toxic effect of the virus per se22.
Corneal graft rejection may be due, in part,
to failure to invoke ACAID. Streilein at al23 not
only discovered that the immunosuppressive
effects of the cornea were abolished in corneas
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Table 7:
MHC and ocular cell type8-13
Ocular cell type MHC class I MHC class II
Corneal epithelium •
Corneal stroma •
Corneal endothelium •
Trabecular meshwork •
Pigmented and non pigmented •
cells of ciliary body
Anterior iris •
RPE cells •
Corneal limbus •
Iris and ciliary body •
Uveoscleral pathway •
Ora serrata •
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31
that had ACAID removed via cauterisation or
keratoplasty but also abolished in corneas that
had been denervated.
Ocular immunology
Anterior segment immunology may be
sub-divided into the following aspects:
• Tear film
• Corneal immunology
• Conjunctival immunology
• Scleral immunology
• Uveal immunology
Tear film
a) Mucous layer
This layer is produced both by the conjunctival
goblet and epithelial cells. The glycocalyx
synthesised by the corneal epithelial cells serves
to attach the mucous layer and in doing so binds
to immunoglobulin in the aqueous24. It has been
suggested that the latter immunological sign may
have antiviral effects. The evidence to support
this is that certain intestinal mucosa, which have
similar binding properties to those seen in the
eye, are able to exert an inhibitory action against
viral replication25.
b) Aqueous layer
The aqueous layer contains the following
antimicrobial factors:
1. Lactoferrin
2. Lysozyme
3. IgA
4. Miscellaneous proteins
1. Lactoferrin
Lactoferrin is produced via the acinar cells of the
lacrimal gland. Its main function is to bind iron,
which is required for bacterial growth. It
therefore possesses both bacteriostatic and
bacteriocidal properties. Furthermore, it is able
to enhance the effects of certain
immunoglobulins.
2. Lyszome
Produced by type A cells in the lacrimal gland,
lysozome constitutes up to 25% of the total tear
protein. It is surprising somewhat that despite
being effective at lysing gram positive bacterial
cell walls, staphylococcus aureus appears
recalcitrant to its action26. However, it is
instrumental in enhancing IgA against gram
negative bacteria.
3. IgA
This is the predominant isotype found in the
human tears in the non-inflamed eye. Very little
is present in serum with the majority secreted
through epithelial cells of structures such as the
lacrimal gland and the lactating breast. The IgA
present in the tears is produced by plasma cells
situated underneath the secretory cells lining the
acini in the lacrimal gland.
IgA is very effective at binding microbes and,
as such, prevents the microbe from adhering to
the mucosal surface. The antibody achieves this
either by interfering with the binding site
directly or by agglutination. Successful binding of
the microorganism enables the tears to wash
them away. IgA possesses other functions
including opsonisation and inactivation of
various bacterial enzymes and toxins. In
addition, it may be involved in antibody
dependent cell-mediated cytotoxicity. It must be
emphasised, however, that IgA is unable to bind
to complement and, as such, is not involved in
the classical pathway27.
4. Miscellaneous proteins
β-Lysin, a bacteriocidal cationic protein, is
present both in the tears and aqueous humour28.
Its bacteriocidal properties are conferred
through their ability to disrupt the microorganisms’
walls. Various components of
complement are present in varying degrees
depending on whether the eye is closed or not.
Closed eye vs open eye
Open eye
In addition to the components of complement
mentioned above, the tears are rich in lysozyme,
lactoferrin and lipocalin (tear pre albumin)
together with low levels of IgA29,30.
Closed eye
In the closed eye environment, levels of IgA and
complement components are increased. In
addition, neutrophils are also recruited. The eye
can be interpreted as being in a state of
subclinical inflammation. However, the eye is
protected from damage induced by either
complement or neutrophils by DAF (Decayed
Accelerating Factor [an inhibitory component of
the complement system])31 and α1 –antitrypsin33.
Corneal immunology
Paradoxically, the cornea, an immune privileged
site, is capable of evoking an immunological
response evidenced by the presence of
subepithelial infiltrates, keratic precipitates,
epithelial and stromal keratitis under certain
clinical conditions.
Cytokine release
Numerous cytokines are synthesised in the
cornea including IL-1, IL-6, IL-8, TNF-α, IFN-γ,
C5a and prostaglandins33. Phagocytosis of certain
antigens via corneal epithelial cells initiate the
release of IL-1 which, in turn, gives rise to the
recruitment of Langerhans’ cells from the limbus
to the central cornea. It is worthy of note that
the same said APC may be recruited centrally in
response to chemical or localised damage34,35.
Investigators have discovered that stromal
fibroblasts synthesise
IL-8 in response to infection by the herpes
simplex virus36. Furthermore, it seems plausible
that the latter cytokine release may be
responsible for the neutrophilic infiltration
observed in cases of herpetic keratitis.
Immunoglobulins
The three isotypes detected in the cornea are
IgM, G and A. IgG predominates in the central
cornea37. By contrast, IgM predominates in the
limbal region. The latter antibody is restricted
from the central cornea owing to its large size.
Antibodies in the cornea are found in the
stroma since they are cationically charged. Due
to their positive ionic charge, they bind to
proteoglycans and the anionic
glycosaminoglycans38.
Antigen/antibody complexes may sometimes
be observed in the corneal stroma in a variety
of pathological conditions (e.g. herpes simplex
keratitis), and because of their ringed shaped
appearance are known as “Wessley rings”.
Complement
The elements of complement found in the
cornea include C1-739 in addition to the
regulatory proteins H and I and C1 inhibitor40.
Interestingly, C1 is present in high
concentrations in the limbus and is restricted
from the central cornea. This finding is
significant since the limbal region is susceptible
to ulceration following complement activation
and immune complex deposition. It has been
suggested that C1 is produced by corneal
fibroblasts whereas the remaining components
detected in the cornea appear to be derived
from the plasma via linked vessels.
Conjunctival immunology
The conjunctival associated lymphoid tissue
(CALT) is part of the more general mucosa
associated lymphoid tissue (MALT). Langerhans’
cells and lymphocytes exist within the
conjunctival epithelial layer. In the substantia
propria, neutrophils, lymphocytes, IgA and IgG,
dendritic cells and mast cells all reside. It is
noteworthy that eosinophils and basophils are
not present in the healthy conjunctiva.
Langerhans’ cells and dendritic cells present
the antigenic peptide to the conjunctival T
helper cells. Following antigenic presentation,
the T cells secrete the cytokine IFN-γ which
serves to promote antigen elimination by
macrophages. This is the delayed-type
hypersensitivity (DTH) response (see later) and
is characteristic of conjunctival pathology such
as phlyctenulosis.
Scleral immunology
There exists only a small number of immune
cells in the sclera compared to the conjunctiva
since it is relatively avascular. In its resting
state, IgG appears to be present in large
amounts41. However, a sclera under stress, may
become immunologically active as a result of
migrating cells from the overlying episcleral and
underlying choroidal vasculature42,43.
Uveal immunology
The uveal tract is an important site from an
immunological perspective for two reasons:
first, it is highly vascular in nature; and second,
the majority of vessels are highly fenestrated
which facilitates the recruitment of leukocytes
during the inflammatory cascade. The uvea
contains numerous cellular components of the
immune system including macrophages, mast
cells, lymphocytes and plasma cells in addition
ot
32 www.optometry.co.uk
to an appreciable number of immune factors.
Although IgG and IgM have been detected in
both the choroid and iris, they exist in greater
numbers in the former structure. The reason for
such disparate levels is the dearth of anionic
antibody binding sites on the iris surface44. By
contrast, the ciliary body harbours tremendous
amounts of IgG by virtue of its anionic tissue
sites.
Immunopathology and the eye
Immunopathology encompasses both pathology
as a result of an over-active immune system
(hypersensitivity and autoimmunity) together
with that acquired through an individuals
inability to fight off infection – namely
immunodeficiency. Unfortunately, it is far
beyond the scope of this article to describe the
latter and its ophthalmic correlates.
In this section, the classification system
pertaining to hypersensitivity reactions will be
described and each subtype with an anterior
segment manifestation will be discussed. The
association between HLA and systemic and
ophthalmic disease will be illustrated together
with a brief overview of the concepts pertaining
to autoimmunity.
Hypersensitivity
The term hypersensitivity refers to the process
whereby the adaptive immune response overreacts
to a variety of infectious and inert
antigens resulting in damage to the host tissue.
Five types of hypersensitivity reactions exist –
all of which vary in their timing following
contact with the antigen (Table
.
• Type I hypersensitivity: allergy
Allergies may affect approximately 17% of the
population45. The term atopic is used to describe
those individuals who possess a genetic
predisposition to allergy. Allergies may occur to
otherwise innocuous antigens (known as
allergens) and infectious agents, e.g. worms.
Type I hypersensitivity exists in two phases, the
sensitisation and effector phases.
Firstly, a harmless allergen causes production
of IgE antibody on first exposure. This IgE
February 8, 2002 OT
diffuses throughout the body until it comes into
contact with mast cells and basophils. Both
these cell types have receptors for IgE antibody.
Although the patient experiences no symptoms
after the initial binding, reintroduction of the
antigen/allergen induces the production of
more IgE and, furthermore, increase the
likelihood of cross-linking with existing
antibodies on the mast cell surface. Such crosslinking
induces the mast cell to degranulate and
release a host of inflammatory mediators such
as histamine, prostaglandins and bradykinin.
Histamine characteristically causes the itchy
symptoms experienced by patients and as a
result of binding to H1 receptors in the eye,
induces vasodilation and enhances mucous
secretion by the goblet cells. Bradykinin
augments vascular permeability, decreases
blood pressure and contracts smooth muscle.
Prostaglandins are also powerful inflammatory
mediators.
Ocular correlates:
The following anterior segment conditions are
due, if not only in part, to type I
hypersensitivity:
• Seasonal allergic conjunctivitis (type I)
• Giant papillary conjunctivitis (types I and IV)
• Vernal keratoconjunctivitis (VKC)
(type I and IV)
• Atopic keratoconjunctivitis (AKC)
(types I and IV) (Figure 7)
Seasonal allergic conjunctivitis can be easily
recognised by chemosis and hyperaemia of the
conjunctiva, lid swelling and excessive
lacrimation. The cornea is not affected.
GPC is well known to optometrists as an
allergic response to contact lenses, prosthetic
lenses, protruding corneal sutures and scleral
buckles. Histological examination of eyes
suffering from GPC have revealed degranulated
mast cells (type I hypersensitivity)46 and CD4+ T
cells (type IV)47, thus corroborating the theory
that such a condition is mediated by both types
of hypersensitivity.
Vernal conjunctivitis can present either in a
palpebral form characteristically exhibiting
giant cobblestone papillae, or in a limbal form
with gelatinous deposits known as Trantas’-dots,
which represent degenerating epithelial cells and
eosinophils. The first corneal change is a
punctate epithelial keratitis, which if left
unchecked develops into a macroerosion
(Figure and finally a corneal plaque develops
(Figure 9). The conjunctiva itself is
characteristically oedematous and contains an
array of immunological cells including
lymphocytes and mast cells. Evidence of type I
involvement includes the histological detection
of, inter alia, degranulated mast cells,
eosinophils and increased levels of IgE in
affected eyes49. The detection of CD4+ T cells and
macrophages is indicative of a delayed
inflammatory component49.
Table 8:
Hypersensitivity reactions
Hypersensitivity Appearance Mediators Mechanism
type time
I. Immediate 2-30 mins IgE Mast cell response
(enhance acute inflammation)
II. Cytotoxic 5-8 hours IgM and IgG Antibody and complement
III. Immune 2-8 hours IgM and IgG Antibody/antigen
complex immune complexes complexes
IV. Delayed type 24-72 hours CD4 and CD8 T cells, T cell mediated
APCs Macrophages activated
Include granulomatous reactions
V. Stimulatory Autoantibodies Autoantibodies against hormone
Figure 8:
Epithelial macroerosion in VKC
Figure 9:
Corneal plaque in VKC
Figure 7:
Corneal thinning and vascularisation in AKC,
excess mucous is present
www.optometry.co.uk 33
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Type II hypersensitivity: cytotoxic
This classification of hypersensitivity involves
either IgG or IgM antibodies, which may induce
cellular lysis due to the involvement of the
classical complement pathway (as seen in blood
transfusion reactions) or recruit and activate
inflammatory cells via complement. The
components of complement include the C5a,
which serves to attract inflammatory cells to the
site of interest. The hypersensitivity reaction is a
result of the excessive amount of extracellular
mediators released by the inflammatory cells to
antigens that are too big to be completely
phagocytosed.
Antibodies to self-antigens, such as the
acetylcholine receptor in myasthenia gravis, is
not only another example of type II
hypersensitivity but also an example of
autoimmune disease.
Ocular manifestations
• Mooren’s ulcer
• Cicatrical pemphigoid
Mooren’s ulcer is a rare peripheral ulcerative
keratitis that exists either as a unilateral,
non-progressive form which has a predilection
for elderly patients or as a more severe,
progressive form affecting both eyes of relatively
young individuals. The signs range from a small
patch of grey infiltrate near the margin to frank
ulceration involving the entire corneal
circumference and, in some cases, the central
region as well. The healing process results in a
thin, vascularised, opaque cornea. Investigators
have identified a significant number of
lymphocytes, neutrophils and plasma cells50 in
the cornea, thus providing unequivocal evidence
to support the theory that this condition has an
immunopathological aetiology.
Cicatrical pemphigoid is a chronic, blistering
disease, which has a predilection for both the
ocular and oral mucous membranes. Unlike its
self-limiting counterpart, pemphigus vulgaris,
cicatrical pemphigoid rarely affects the skin.
Ocular cicatrical pemphigoid is a serious,
bilateral condition that represents effective
shrinkage of the conjunctiva. Although the
initial presentation may be subacute and
non-specific, it frequently progresses to
symblepharon, entropion with secondary
trichiasis, dry eye, ankyloblepharon and
conjunctival fornix shortening. There is evidence
to support the presence of IgG antibodies
directed against self-antigen in the basement
membrane of both the skin and eye51. Binding of
the aforementioned antibodies may activate
complement with subsequent recruitment of
inflammatory cells into the area. The process of
cicatrisation is achieved through the secretion of
collagen via fibroblasts as a result of stimulation
via cytokines released from the invading
inflammatory cells.
Type III hypersensitivity:
immune complex
Large pathogens with multiple antigenic sites
have several antibodies bound to them forming
immune complexes. Normally, these complexes
are removed by the mononuclear phagocytes in
the liver and spleen with no adverse sequelae.
However, persistence of immune complexes does
occur in certain individuals leading to their
deposition in tissue. As a consequence of the
latter action, complement may be activated thus
paving the way for inflammatory cells to enter
the deposition site. Since blood vessels (which
filter plasma at high pressure and exhibit a great
deal of tortuosity) are more susceptible for
immune complex deposition, the ciliary body is
particularly vulnerable to this type of
hypersensitivity reaction.
Ocular manifestations
• Uveitis (Crohn’s disease)
• Peripheral corneal lesions associated with
rheumatoid arthritis
• Stevens Johnson syndrome
• Sjögren’s syndrome
The signs and symptoms of the above will be
described in future articles in the series.
Type IV hypersensitivity: delayed-type
hypersensitivity (DTH)
The term DTH has been used to describe such a
reaction owing to its prolonged time-scale
relative to the other hypersensitivity types.
Although DTH can be transferred by T cells that
have been previously sensitised by an antigen, it
cannot be transferred in serum.
The sequence of events leading to DTH begins
with initial presentation of the antigen peptide
to T cells by APCs (e.g. Langerhans’ cells). The
primed T cell migrates to the site of antigenic
entry whereby it releases pro-inflammatory
mediators such as TNF. The release of these
cytokines facilitates blood flow and extravasation
of plasma contents to the area. The activation of
CD4 T helper and CD8 cells results in the release
of IFN-γ and, as a consequence, enhances
macrophage activity in that area. Resolution of
DTH is dependent on the efficacy with which
such phagocytes can remove the offending
antigen.
Recalcitrant infectious agents result in a
chronic DTH that causes the chronically activated
macrophages to fuse together and form
multinucleated giant cells. In an attempt to
contain the infectious agent, macrophages may
undergo further inter connections to resemble an
epithelial layer. Owing to the similarity to this
layer they are referred to as epitheloid cells.
Both epitheloid and multinucleated giant cells
secrete factors that induce fibrosis resulting in
granuloma formation. Thus granulomata are the
hallmark of chronic inflammation. Damage and
loss of function of the neighbouring tissues
frequently ensues until the agent is removed
either chemically or surgically.
Ocular manifestations
• Ocular allergies (VKC, AKC GPC)
• Idiopathic uveitis
• Sympathetic ophthalmia
• Phlyctenulosis
Type V hypersensitivity
This relatively new category encompasses the
concept of autoantibodies binding to hormone
receptors that mimic the hormone itself. This
results in stimulation of the target cells.
Examples include thyrotoxicosis.
Autoimmunity
The ability to react against self-antigens is
known as autoimmunity. However, a significant
number of people exist who harbour autoantibodies
and yet remain asymptomatic. The
corollary of this is that the presence of
autoreactive cells per se is not sufficient to
trigger autoimmune disease. In fact,
autoimmune disease is a result of breakdown of
one of the immunoregulatory mechanisms.
Furthermore, autoimmune disease may be
classified as either being organ specific (e.g.
insulin dependent diabetes mellitus, Grave’s
disease) or non-organ specific (e.g. Sjögren’s
syndrome, ankylosing spondylitis).
It is important to realise that the causes of
autoimmune diseases are multifactorial. The
main predisposing factors are age, gender,
infection and genetics. However, one of the
most important factors of interest to
immunologists and clinicians alike is the
association between HLA and autoimmune
disease. When determining the likelihood of
contracting a disease both epidemiologists and
clinicians refer to the relative risk. In the case of
HLA antigen, the relative risk compares the
chance of a person who has a particular HLA
antigen acquiring a disease to those individuals
who do not have such an antigen. The
association is exemplified by the relative risk of
suffering from ankylosing spondylitis (AS) in
individuals who possess HLA-B27. In patients
suffering from AS, the prevalence of HLA-B27 is
90% and this figure rises to 95% in patients
who suffer both with the disease and acute
iritis. Indeed, in the UK approximately 45% of
patients who present with acute iritis will
harbour HLA-B2752.
It is therefore important that patients who
present with anterior uveitis are screened for
HLA-B27 because although a positive result may
not necessarily be diagnostic, its presence will
certainly improve the sensitivity of further
radiological tests.
Table 9 compares the HLA associations with
both ophthalmic disorders together with those
systemic disorders relevant to the ophthalmic
practitioner.
Conclusion
This article has only broached the fascinating
subject of ocular immunology. A basic
understanding of immunology is required if
practitioners are to therapeutically manage their
patients. Further articles in the series will help
to reinforce the concepts of this challenging
subject.
Acknowledgement
The author would like to thank Professor Roger
Buckley for his permission to use Figures 7 to 9.
ot
34 February 8, 2002 OT www.optometry.co.uk
Figures 1 to 6 reprinted from
Immunology, Third Edition, Roitt,
Brostoff and Male, 1993 by
permission of the publisher
Mosby.
About the author
Gregory Heath is an optometrist
working part-time in private
practice. He was recently awarded
the diploma in clinical optometry
at City University and is currently
reading medicine at the Royal
Free and University College,
London, Medical School.
References
References are available upon
request. Please fax 01252-816176
or email info@optometry.co.uk.
Table 9:
HLA and ophthalmic disease
Disease HLA Relative risk 54 Ocular
association manifestation
Ankylosing spondylitis B27 90 Anterior uveitis
Reiter’s disease B27 33 Mucopurulent conjunctivitis
Anterior uveitis
Keratitis
Rheumatoid DR4 7 Keratoconjunctivitis sicca
Keratitis
Scleritis
Primary Sjögrens’ syndrome DR3, DR5 10 Keratoconjunctivitis sicca
Sarcoidosis DR3 Not known Anterior uveitis (acute and chronic)
Dacryoadentis
Retinal vasculitis,
neovascularisation,
Optic nerve granulomata
Cicatrical pemphigoid DQw7 Not known Shrinkage of conjunctiva
Behcet’s disease B5 3 Anterior uveitis
Retinitis, periphlebitis,
retinal oedema
Sympathetic DR4, A11, Not known Panuveitis
ophthalmia B40
Systemic lupus DR2, DR3 3 Punctate epithelial keratopathy
erythematosus Keratopathy
Necrotising scleritis
Retinal lesions (cotton wool spots)
Autoimmune optic neuropathy
1. Which one of the following is not part of the
innate immune system?
a. Mast cells
b. Complement
c. Phagocytes
d. T cells
2. Which one of the following statements is
correct regarding the innate immune system?
a. It is specific
b. It evokes a more potent response on
secondary exposure
c. It represents the first line of defence
d. It is able to memorise pathogens on
subsequent exposures
3. Which one of the following statements is
correct regarding the cells of the innate
system?
a. Basophils are important phagocytes
b. During phagocytosis the pathogen becomes
initially internalised as a phago-lysosome
c. Eosinophils play an important role in
combating virally-infected cells
d. Langerhans’ cells form a bridge between
innate and adaptive immunity
4. Which one of the following statements is
correct regarding the adaptive immune
system?
a. It consists of all types of lymphocytes
b. T cells produce antibodies
c. T cells maturate in the thymus
d. B cells are produced in the spleen
5. Which one of the following statements is
correct regarding T cells?
a. T cells can be subdivided into TH1 and TH2
subtypes only
b. T cells alone can identify any type of antigen
c. T cells express cell surface proteins denoted by
cluster determinant (CD) numbers
d. All T cells are involved in initiating the
inflammatory response
6. Which one of the following statements is
incorrect?
a. B cells have antibodies as their cell surface
receptor
b. There are five types of antibody
c. IgE is an important antibody in allergies
d. All B cells differentiate into plasma cells
7. Which one of the following statements is
correct regarding ocular immune privilege?
a. There is an absence of Langerhans’ cells in the
central cornea
b. Aqueous humour has no role
c. Abundant vascular supply is vital
d. All ocular cells express MHC class II
8. Which one of the following statements
concerning ocular immunology is incorrect?
a. Levels of IgA and complement
increase when the eyes are closed
b. IgA is the predominant antibody
in blood and tissue fluid
c. IgM, IgG and IgA antibody isotypes have been
identified in the cornea
d. The sclera contains a smaller number of
immune cells than the conjunctiva
9. In a patient suffering from vernal conjunctivitis,
which one of the following statements is
correct?
a. Trantas’-dots represent infiltrating T cells
b. Affected eyes have increased levels of IgD
c. Histologically, mast cells, lymphocytes and
macrophages have been identified
d. Is due to type III Hypersensitivity
10. Which one of the following statements is
incorrect?
a. HLA-B27 is a risk factor for both anterior uveitis
and ankylosing spondylitis
b. Granulomas are present in type IV
hypersensitivity reactions
c. Histamine is an important vasoconstrictor
d. IgE mediated hypersensitivity is of rapid onset
11. What proportion of patients with acute iritis will
harbour HLA-B27?
a. 15%
b. 25%
c. 45%
d. 65%
12. Which one of the following statements is correct
regarding a type I hypersensitivity reaction?
a. It always occurs in isolation
b. It is characterised by the presence of
macrophages
c. It is associated with myasthenia gravis
d. It involves the degranulation of mast cells
following the cross-linking of IgE bound to its
cell surface
Multiple choice questions - Basic immunology Please note there is only one correct answer
An answer return form is included in this issue. It should be completed and returned to: CPD Initiatives (c4082c),
OT, Victoria House, 178–180 Fleet Road, Fleet, Hampshire,
The immune system is the body’s natural defence in combating organisms.
Immunology has developed rapidly over the past decade owing to the refinements made
in the molecular tests employed in this area of research. Therefore, the keen reader is
encouraged to peruse the ophthalmic and immunological literature in order to keep
abreast of the latest developments in this field.
The College of
Optometrists has
awarded this article 2
CET credits. There are
12 MCQs with a pass
mark of 60%.
Owing to the complex nature of this subject, it is
far beyond the scope of this article to cover all
aspects of immunology. Rather, the aims of the
article are twofold: first to acquaint the busy
practitioner with the basic concepts of the
immune system; and second, to introduce the
reader to the more specific topic of ocular
immunology – the study of the ocular immune
system.
Finally, since it is envisaged that optometrists
will one day prescribe therapeutic agents, the
discussion is limited to the anterior segment and
anterior uvea.
Innate & adaptive immune systems
The immune system can be thought of as having
two “lines of defence”: the first, representing a
non-specific (no memory) response to antigen
(substance to which the body regards as foreign
or potentially harmful) known as the innate
immune system; and the second, the adaptive
immune system, which displays a high degree of
memory and specificity. The innate system
represents the first line of defence to an
intruding pathogen. The response evolved is
therefore rapid, and is unable to “memorise” the
same said pathogen should the body be exposed
to it in the future. Although the cells and
molecules of the adaptive system possess slower
temporal dynamics, they possess a high degree
of specificity and evoke a more potent response
on secondary exposure to the pathogen.
The adaptive immune system frequently
incorporates cells and molecules of the innate
system in its fight against harmful pathogens.
For example, complement (molecules of the
innate system - see later) may be activated by
antibodies (molecules of the adaptive system)
thus providing a useful addition to the adaptive
system’s armamentaria.
A comparison of the two systems can be seen
in Table 1.
ot
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Gregory Heath BSc (Hons), MCOptom, Dip. Clin. Optom
ABDO has awarded this
article
2 CET credits (GD).
26 February 8, 2002 OT
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a
Cells of the innate
immune system
Phagocytes
Although sub-divided into two main types,
namely neutrophils and macrophages, they
both share the same function - to engulf
microbes (phago - I eat, Latin).
Neutrophils
Microscopically, these cells possess a
characteristic, salient feature - a
multilobular nucleus (Figure 2). As such,
these cells have been referred to as
polymorphonuclear leukocytes (PMNs) and
have a pivotal role to play in the
development of acute inflammation. In
addition to being phagocytic, neutrophils
contain granules and can also be classed as
one of the granulocytes. The granules
contain acidic and alkaline phosphatases,
defensins and peroxidase - all of which
represent the requisite molecules required for
successful elimination of the unwanted
microbe(s).
Macrophages
Macrophages (termed monocytes when in the
blood stream) have a horseshoe-shaped nucleus
and are large cells. Properties of macrophages
include phagocytosis and antigen presentation
to T cells (see later). Unlike neutrophils (which
are short-lived cells), they are seen in chronic
inflammation as they are long-lived cells.
Mononuclear phagocytic system
The cells comprising the monocyte phagocytic
system are tissue bound and, as a result, are
further sub-divided depending on their location.
A list of the cells together with their
corresponding location can be found in Table 2.
Table 1: Cells and molecules of the innate and adaptive immune systems
Immunity Cells Molecules
Innate Natural killer (NK) cells Cytokines
Mast cells Complement
Dendritic cells Acute phase proteins
Phagocytes
Adaptive T and B cells Cytokines
Antibodies
Components of the immune system can be seen in Figure 1.
Figure 1 The principle components of the immune system are listed, indicating which cells
produce which soluble mediators. Complement is made primarily by the liver, with some
synthesised by mononuclear phagocytes. Note that each cell only produces a particular set of
cytokines, mediators etc
Figure 2 Morphology of the neutrophil. This
shows a neutrophil with its characteristic
multilobed nucleus and neutrophilic granules
in the cytoplasm. Giemsa stain, x 1500
www.optometry.co.uk 27
Table 2: Examples of cells of the
mononuclear phagocytic system and their
respective locations
Cells Location
Monocytes Blood stream
Alveolar macrophages Lungs
Sinus macrophages Lymph nodes
and spleen
Kupffer cells Liver
Dendritic cells
Dendritic cells consist of Langerhans’ and
interdigitating cells and form an important
bridge between innate and adaptive immunity,
as the cells present the antigenic peptide to the
T helper cell (adaptive immunity). Such cells are
therefore known as professional antigen
presenting cells (APCs). Table 3 illustrates the
various types of dendritic cells together with an
example of their location.
Eosinophils
Eosinophils (so called because their granules
stain with eosin – Figure 4) are granulocytes
that possess phagocytic properties. Despite the
fact that they represent only 2-5 % of the total
leukocyte population, they are instrumental in
the fight against parasites that are too big to be
phagocytosed.
Phagocytosis - the process
Phagocytosis is the process by which cells engulf
microorganisms and particles (Figure 3). Firstly,
the phagocyte must move towards the microbe
under the influence of chemotactic signals, e.g.
complement (see later). For the process to
continue, the phagocyte must attach to the
microbe either by recognition of the microbial
sugar residues (e.g. mannose) on its surface or
complement/antibody, which is bound to the
pathogen. Following attachment, the
phagocyte’s cell surface invaginates and the
microbe becomes internalised into a
phagosome. The resultant phagosome fuses with
multiple vesicles containing O2 free radicals and
other toxic proteins known as lysosomes to form
a phagolysosome. The microbe is subsequently
destroyed.
Opsonisation (“to make tasty” - Greek)
Opsonins are molecules, which enhance the
efficiency of the phagocytic process by coating
the microbe and effectively marking them for
their destruction. Important opsonins are the
complement component C3b and antibodies.
Natural killer (NK) cells
NK cells are also known as “large granular
lymphocytes” (LGLs) and are mainly found in the
circulation. They comprise between 5-11% of the
total lymphocyte fraction. In addition to
possessing receptors for immunoglobulin type G
(IgG), they contain two unique cell surface
receptors known as killer activation receptor and
killer inhibition receptor. Activation of the
former initiates cytokine (“communication”)
molecules from the cell whilst activation of the
latter inhibits the aforesaid action.
NK cells serve an important role in attacking
virally-infected cells in addition to certain
tumour cells. Destruction of infected cells is
achieved through the release of perforins and
granyzymes from its granules, which induce
apoptosis (programmed cell death). NK cells are
also able to secrete interferon-γ (IFN-γ). This
interferon serves two purposes: first, to prevent
healthy host cells from becoming infected by a
virus; and second, to augment the T cell
response to other virally infected cells
(see later).
Mast cells and basophils
Morphologically, mast cells and basophils are
very similar in that both contain electron dense
granules in the cytoplasm. Basophils are
so-called owing to the fact that their granules
stain with a basic dye. Unlike mast cells, which
are present in close proximity to blood vessels in
connective tissue, basophils reside in the
circulation.
Both cell types are instrumental in initiating
the acute inflammatory response. Degranulation
is achieved either by binding to components of
the complement system or by cross-linking of
the IgE antibody which results in the release of
pro-inflammatory mediators including histamine
and various cytokines. The former induces
vasodilation and augments vascular permeability
whilst the latter are important in attracting
both neutrophils and eosinophils.
Table 3: Dendric cells and location
Cells Location
Langerhans cell Limbus, skin
Interdigitating cell T cell areas in
lymph nodes
Figure 4 Morphology of the eosinophil. The
multilobed nucleus is stained blue and the
cytoplasmic granules are stained red.
Leishman stain, x 1800
Figure 3
Phagocytes arrive at a site of inflammation
by chemotaxis. They may then attach to
microorganisms via their non-specific cell
surface receptors. Alternatively, if the
organism is opsonised with a fragment of
the third complement component (C3b),
attachment will be through the phagocyte’s
receptors for C3b. If the phagocyte
membrane now becomes activated by the
infectious agent, it is taken into a
phagosome by pseudopodia extending
around it. Once inside, lysosomes fuse with
the phagosome to form a phagolysosome
and the infectious agent is killed. Undigested
microbial products may be released to the
outside
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February 8, 2002 OT www.optometry.co.uk
for the successful eradication of an invading
virus by the innate immune system.
Type II IFN, IFN-γ, is produced by T Helper
cells and NK cells and is able to augment both
the antigen presenting properties together with
the phagocytic properties of the APCs (e.g.
macrophages and dentritic cells).
Adaptive immunity
As mentioned previously, there is a great deal of
synergy between the adaptive immune system
and its innate counterpart. The adaptive immune
system comprises two main types of leukocyte
known as B and T lymphocytes. Before describing
these important cell types, it is necessary to
acquaint the reader with both the primary and
secondary lymphoid organs and tissues in the
body. These are summarised in Table 4.
The bone marrow represents the dominant site
for haemopoiesis (production of blood cells and
platelets). Although most of the haemopoietic
cells maturate in this region, T lymphocytes do
so in the thymus. In the thymus, premature T
cells undergo a process of positive and negative
selection whereby the former are allowed to
progress to maturity whilst the latter are marked
for termination via apoptosis (see central
tolerance).
Lymphocytes
Morphologically, there are three types of
lymphocytes: T, B and NK cells. However, only T
and B lymphocytes exhibit memory and
specificity and, as such, are responsible for the
unique quality of the adaptive immune system.
Resting B lymphocytes are able to react with
free antigen directly when it binds to their cell
surface immunoglobins which act as receptors. T
lymphocytes do not react with free antigen and
instead make use of APCs to phagocytose the
antigen and then to express its component
28
Molecules of the innate
immune system
There are many molecules, which work in concert
with the cells of the innate immune system and
which also foster close functional links with their
adaptive counterpart. The three major molecules
are:
• Complement
• Acute phase proteins (APP)
• Interferons (IFNs)
Complement
The complement system represents a large group
of independent proteins (denoted by the letter C
and followed by a number), secreted by both
hepatocytes (liver cells) and monocytes.
Although these proteins maybe activated by both
the adaptive immune system (classical pathway)
or innate immune system (alternative pathway),
the nomenclature is derived from the fact that
the proteins help (“complement”) the antibody
response.
Activation of complement via the microbe
itself is known as the alternative pathway. The
classical pathway requires the interaction of
antibody with specific antigen. The C3
component is the pivotal serum protein of the
complement system. Binding of the antigen to
C3 results in two possible sequelae. In either
case, C3 component becomes enzymatically
converted to C3b. The bacterial cell wall can
either remain bound to C3b and become
opsonised (since phagocytes have receptors for
C3b) or act as a focus for other complement
proteins (namely C5, 6, 7, 8 and 9). The latter
form the membrane attack complex (MAC), which
induces cellular lysis.
The functions of the complement system may
be summarised as follows:
• Opsonisation
• Lysis (destruction of cells through damage/
rupture of plasma membrane)
• Chemotaxis (directed migration of immune
cells)
• Initiation of active inflammation via direct
activation of mast cells
It is important that complement is regulated to
protect host cells from damage and/or their total
destruction. This is achieved by a series of
regulatory proteins, which are expressed on the
host cells themselves.
Acute phase proteins
These serum proteins are synthesised by
hepatocytes and are produced in high numbers
in response to cytokines released from
macrophages.
Interferons (IFNs)
IFNs are a group of molecules, which limit the
spread of viral infections (Figure 5). There are
two categories of IFNs, namely type I and type II.
Type I IFNs maybe sub-divided further into IFN-α
and β. IFN-γ is the sole type II interferon. Type I
IFNs are induced by viruses, pro-inflammatory
cytokines and endotoxins from gram negative
bacterial cell walls. Their presence remains vital
proteins on the cell surface adjacent to special
host proteins called major histocompatibility
complex (MHC) class II molecules. As discussed,
antigen presenting cells which express MHC class
II molecules include dendritic cells and
macrophages. This “afferent” phase must occur
in order for the T cell to recognise the antigen.
The “efferent” phase occurs when activated
lymphocytes enter the tissue and meet antigen
again. This results in multiplication and secretion
of cytokines or immunoglobins in order to
destroy the antigen.
T cells
T cells can be broadly divided into both T helper
(TH) and cytotoxic T cells (Tc). Furthermore, TH
cells may be sub-divided into TH1 and TH2. The
former are pro-inflammatory T cells and
stimulate macrophages whilst the latter
orchestrate B cell differentiation and maturation
and hence are involved in the production of
humoral immunity (antibody mediated). T cells
express cell surface proteins, described by cluster
determination (CD) numbers. TH cells express CD4
molecules on their cell surface, which enable the
lymphocyte to bind to a MHC class II molecule.
The T cell receptor is unique in that it is only
able to identify antigen when it is associated
with a MHC molecule on the surface of the cell.
Cytotoxic T cells are primarily involved in the
destruction of infected cells, notably viruses.
Unlike TH cells, cytotoxic cells possess CD8 cell
surface markers, which bind to antigenic
peptides expressed on MHC class I molecules.
B cells and antibodies
(immunoglobulins - Ig)
B cells are lymphocytes that produce antibodies
(immunoglobulins) and can recognise free
antigen directly. They are produced in the bone
marrow and migrate to secondary lymphoid
organs. B cells are responsible for the
Figure 5
When host cells become infected
by virus, they may produce
interferon. Different cell types
produce interferon-α (IFN-α ) or
interferon-β (IFN-β); interferon-γ
(IFN-γ) is produced by some types
of lymphocyte (TH) after activation
by antigen. Interferons act on other
host cells to induce a state of
resistance to viral infection. IFN-γ
has many other effects as well
Table 4:
Primary and secondary lymphoid organs
Primary lymphoid organs Secondary lymphoid organs
Bone marrow Lymph nodes
MALT - mucosa associated lymphoid
tissue (includes bronchus, gut, nasal and
conjunctival associated mucosal tissues)
Spleen
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Module 4 Part 2
29
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a
development of antibody mediated immunity
known as humoral mediated immunity.
When activated by foreign antigen, B cells
undergo proliferation and mature into antibody
secreting plasma cells. The latter are rich in
organelles such as rough endoplasmic reticulum
and mitochondria, which confer their ability to
secrete soluble proteins (antibodies). Not all
proliferating B cells develop into plasma cells.
Indeed, a significant proportion remain as
memory B cells through a process known as
clonal selection. This process is vital in
eliminating the antigen should the body become
re-exposed to it in the future. T cells are also
clonally selected and this confers to the
production of T memory cells.
Although T and B cells behave differently,
both are able to recirculate around the body
migrating from blood to tissue and vice versa.
The ability to recirculate obviously increases the
efficiency with which cells of the immune system
can home onto the invading antigen.
Antibodies
Antibodies have two roles to play - the first is to
bind antigen and the second is to interact with
host tissues and effector systems in order to
ensure removal of the antigen
(Figure 6).
There are five different types (known as
isotypes) of antibody in the human immune
system - namely IgM, IgG, IgA, IgE and IgD. In
addition, there are four sub classes of IgG
(IgG1-4). The basic antibody unit consists of a
glycosylated protein consisting of two heavy and
two light, polypeptide chains. The region which
binds to the antigen is known as the Fab region,
while the constant region, Fc, not only
determines the isotype but is the region
responsible for evoking effector systems, e.g.
mast cell activation. The term immune complex
refers to the combination of antigen and
antibody and will be discussed later in the article
(see type III hypersensitivity).
The antibody isotypes together with their
corresponding function are illustrated in Table 5.
MHC
Major histocompatability complex (MHC) are cell
surface proteins classified as class I (also termed
human leucocytic antigen [HLA] A, B and C),
found on all nucleated cells and class II (termed
HLA, DP, DQ and DR), found on all antigen
presenting cells (APCs). MHC molecules are the
sine qua non of T cell induced immunity.
Clinically, there is a strong association
between HLA and certain systemic and ocular
diseases (see later).
Cytokines
Cytokines (also termed interleukins [IL] meaning
“between white blood cells”) are small molecules
that act as a signal between cells and have a
variety of roles including chemotaxis, cellular
growth and cytotoxicity. Owing to their ability to
control immune activity, they have been
described as the “hormones” of the immune
system.
Characteristics
Crosses placenta thus providing newborn with useful humoral immunity
High affinity
Predominant antibody in blood and tissue fluid
Large pentameric structure in circulation
Present in monometric form on B cell surface
Secreted form is predominant antibody in early immune
response against antigen
Reaches 75% of adult levels at 12 months of age
Exists in both a monometric and dimeric form
Secretory IgA (dimeric form) represents 1st line of defence against
microbes invading the mucosal surface, e.g. tears
Low levels in circulation
Increased levels in worm infections
Fc region has high affinity for mast cell thus involved in allergy
Antigen receptor on B cells
Absent from memory cells
Table 5:
Antibody isotypes and corresponding functions
Antibody
IgG
IgM
IgA
IgE
IgD
Table 6:
Various cytokines, their sources and functions
Cytokine Source Function
IL-1 Macrophages 1. T, B cell activation
2. Mobilisation of PMNs
3. Induction of acute phase proteins
IL-2 T cells Proliferation of T and NK cells
Il-4 Th2 cells B cell activation
Mast cells IgE response
IL-8 Macrophages Chemotaxis of PMNs
T cells
Fibroblasts
Keratinocytes
IL-10 TH2 cells B cell activation
Macrophages Suppress macrophages
IL-12 B cells Stimulate TH1
Inhibit TH2
TGF β (transforming growth factor) T cells Inhibits other cytokines
TNF α (tumour necrosis factor) Macrophages Inflammation
Figure 6
When a microorganism lacks the
inherent ability to activate
complement or bind to
phagocytes, the body provides
antibodies as flexible adaptor
molecules. The body can make
several million different antibodies
able to recognise a wide variety
of infectious agents. Thus the
antibody illustrated binds microbe
1, but not microbe 2, by its
‘antigen-binding protein’ (Fab).
The ‘Fc portion’ may activate
complement or bind to Fc
receptors on host cells,
particularly phagocytes.
Table 6 summarises some of the cytokines
pertinent to ocular immunology, their functions
and their progenitors. Since interferons have
been discussed earlier in the article, they have
been omitted from the table.
Central and peripheral
tolerance: nature’s way of
containing the immune
response
Since various cells of the immune system are
capable of reacting with self-antigens, it is
therefore essential that the human body has
mechanisms to suppress/eliminate autoreactive
cells. Failure to do so, can, in some cases, lead
to the development of autoimmune diseases
(see later).
Central tolerance
Central tolerance refers to the process whereby
both immature B and T cell lymphocytes, which
react against normal, healthy cells
(self-antigens), are eliminated via apoptosis.
Peripheral tolerance
This involves the removal of mature lymphocytes,
which are not tolerant to healthy cells.
Ocular immune privilege
There are numerous sites in the body whereby
tissue may be grafted with minimal risk of
rejection. Such regions include, inter alia, the
testis, thyroid lens, anterior chamber, cornea,
iris and ciliary body1,2.
It is important that immune privilege is not
simple interpreted as the host’s inability to
initiate an immune response to a transplanted
tissue. Rather, it is an area of the body in which
there exists a paucity of various elements of the
human immune system in response to an
antigen.
Factors
The factors purported by investigators that
contribute to the phenomenon of ocular immune
privilege include:
• Isolation from a vascular supply
• Isolation from a lymphatic supply
• Presence of a vascular barrier
• Ability to suppress the immune response
• Anterior chamber associated immune
deviation (ACAID)
Vascular supply
The healthy cornea is a good example of an
ocular site devoid of a vascular network. The
evidence to support the role a vascular network
has to play in the mechanism of graft rejection
is unequivocal since the risk of failure correlates
positively with the degree of host
vascularisation3.
Vascular barrier
There is a plethora of evidence in the
ophthalmic literature to support the existence of
a blood-ocular barrier. Furthermore, the same
said barrier encompasses different elements
including tight junctions between retinal
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30 February 8, 2002 OT
endothelial cells and the presence of junctional
complexes linking retinal pigment epithelial
cells4.
Lymphatic role
The fact that skin allografts were not rejected
following lymph node removal5 led investigators
to hypothesise that immune privilege was solely
due to the absence of the same said system at a
particular anatomical site. However, although
certain immune privileged sites do indeed lack
lymphatic drainage, others such as the testes6
and eye7 do possess such a system. It appears
that a proportion of the aqueous humour drains
via the uveoscleral pathway into the lymphatic
vessels in the head and neck.
The eye, APCs & MHCs
As mentioned previously, APCs, through their
ability to express MHC class II molecules, are
potent progenitors of the immune response.
Moreover, such cells are capable of activating T
cells within the tissue itself. It is therefore not
unreasonable to assume that a paucity of APCs
may play an important role in immune
privilege. In addition, failure to express MHC
class I molecule would make a tissue immune
against the lytic action of the cytotoxic T cells.
Although the aforementioned mechanisms
are theoretically plausible, cells expressing both
MHC class I and II molecules have been
detected in the eye. Table 7 illustrates the
relationship between histocompatability class
and ocular cell type.
It is noteworthy that the epithelial cells of
the crystalline lens are devoid of class I
expression14 and that the Langerhans’ cells
(class II expression) are absent from the central
cornea15.
It is interesting that not all cells, which
express MHC class II act as professional APCs in
the eye. Indeed, it has been shown that such
cells reside in the iris and ciliary body and not
only fail to present alloantigens to T cells, but
have the ability to suppress mixed lymphocyte
reactions16.
The failure to incite the inflammatory
response has attracted a great deal of interest
amongst ophthalmologists and immunologists
alike. It appears such suppression is achieved by
various factors present in the aqueous humour
(e.g. transforming growth factor - β).
Anterior chamber associated immune
deviation (ACAID)
As a result of experiments with rats,
investigators discovered that antigens placed in
the anterior chamber resulted in systemic
inhibition of delayed type hypersensitivity (DTH
or type IV hypersensitivity) reactions to the
same said antigens17. This phenomenon has been
coined anterior chamber associated immune
deviation (ACAID). The anterior chamber is thus
able to suppress delayed type hypersensitivity
reactions and inhibit the production of
complement fixing antibodies18,19. However, it
has no inhibitory effect on cytotoxic T cell
activity and has a minimal influence on the
production of non-complement fixing
antibodies.
With respect to the endothelium, two
adaptations prevent it from immunological
injury: first, avoidance of cytotoxic T lymphocyte
(CTL)-mediated lysis; and second, inhibition of
DTH responses in the anterior chamber22. It
achieves the former through the cells inability
to express MHC class I molecules21. The corollary
of this, however, is that virally infected cells
may persist in this region. The râison d’etre of
ACAID is to protect the eye from the DTH
response to pernicious antigens. As a result of
its location in relation to the anterior chamber,
the corneal endothelium seems well placed to
reap the benefits of ACAID.
ACAID is beneficial in reducing the incidence
of stromal keratitis in herpes simplex virus
infection. It therefore seems reasonable to
assume that such an unwanted corneal sideeffect
occurs as a result of a DTH response
rather than the toxic effect of the virus per se22.
Corneal graft rejection may be due, in part,
to failure to invoke ACAID. Streilein at al23 not
only discovered that the immunosuppressive
effects of the cornea were abolished in corneas
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Table 7:
MHC and ocular cell type8-13
Ocular cell type MHC class I MHC class II
Corneal epithelium •
Corneal stroma •
Corneal endothelium •
Trabecular meshwork •
Pigmented and non pigmented •
cells of ciliary body
Anterior iris •
RPE cells •
Corneal limbus •
Iris and ciliary body •
Uveoscleral pathway •
Ora serrata •
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Module 4 Part 2
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a
31
that had ACAID removed via cauterisation or
keratoplasty but also abolished in corneas that
had been denervated.
Ocular immunology
Anterior segment immunology may be
sub-divided into the following aspects:
• Tear film
• Corneal immunology
• Conjunctival immunology
• Scleral immunology
• Uveal immunology
Tear film
a) Mucous layer
This layer is produced both by the conjunctival
goblet and epithelial cells. The glycocalyx
synthesised by the corneal epithelial cells serves
to attach the mucous layer and in doing so binds
to immunoglobulin in the aqueous24. It has been
suggested that the latter immunological sign may
have antiviral effects. The evidence to support
this is that certain intestinal mucosa, which have
similar binding properties to those seen in the
eye, are able to exert an inhibitory action against
viral replication25.
b) Aqueous layer
The aqueous layer contains the following
antimicrobial factors:
1. Lactoferrin
2. Lysozyme
3. IgA
4. Miscellaneous proteins
1. Lactoferrin
Lactoferrin is produced via the acinar cells of the
lacrimal gland. Its main function is to bind iron,
which is required for bacterial growth. It
therefore possesses both bacteriostatic and
bacteriocidal properties. Furthermore, it is able
to enhance the effects of certain
immunoglobulins.
2. Lyszome
Produced by type A cells in the lacrimal gland,
lysozome constitutes up to 25% of the total tear
protein. It is surprising somewhat that despite
being effective at lysing gram positive bacterial
cell walls, staphylococcus aureus appears
recalcitrant to its action26. However, it is
instrumental in enhancing IgA against gram
negative bacteria.
3. IgA
This is the predominant isotype found in the
human tears in the non-inflamed eye. Very little
is present in serum with the majority secreted
through epithelial cells of structures such as the
lacrimal gland and the lactating breast. The IgA
present in the tears is produced by plasma cells
situated underneath the secretory cells lining the
acini in the lacrimal gland.
IgA is very effective at binding microbes and,
as such, prevents the microbe from adhering to
the mucosal surface. The antibody achieves this
either by interfering with the binding site
directly or by agglutination. Successful binding of
the microorganism enables the tears to wash
them away. IgA possesses other functions
including opsonisation and inactivation of
various bacterial enzymes and toxins. In
addition, it may be involved in antibody
dependent cell-mediated cytotoxicity. It must be
emphasised, however, that IgA is unable to bind
to complement and, as such, is not involved in
the classical pathway27.
4. Miscellaneous proteins
β-Lysin, a bacteriocidal cationic protein, is
present both in the tears and aqueous humour28.
Its bacteriocidal properties are conferred
through their ability to disrupt the microorganisms’
walls. Various components of
complement are present in varying degrees
depending on whether the eye is closed or not.
Closed eye vs open eye
Open eye
In addition to the components of complement
mentioned above, the tears are rich in lysozyme,
lactoferrin and lipocalin (tear pre albumin)
together with low levels of IgA29,30.
Closed eye
In the closed eye environment, levels of IgA and
complement components are increased. In
addition, neutrophils are also recruited. The eye
can be interpreted as being in a state of
subclinical inflammation. However, the eye is
protected from damage induced by either
complement or neutrophils by DAF (Decayed
Accelerating Factor [an inhibitory component of
the complement system])31 and α1 –antitrypsin33.
Corneal immunology
Paradoxically, the cornea, an immune privileged
site, is capable of evoking an immunological
response evidenced by the presence of
subepithelial infiltrates, keratic precipitates,
epithelial and stromal keratitis under certain
clinical conditions.
Cytokine release
Numerous cytokines are synthesised in the
cornea including IL-1, IL-6, IL-8, TNF-α, IFN-γ,
C5a and prostaglandins33. Phagocytosis of certain
antigens via corneal epithelial cells initiate the
release of IL-1 which, in turn, gives rise to the
recruitment of Langerhans’ cells from the limbus
to the central cornea. It is worthy of note that
the same said APC may be recruited centrally in
response to chemical or localised damage34,35.
Investigators have discovered that stromal
fibroblasts synthesise
IL-8 in response to infection by the herpes
simplex virus36. Furthermore, it seems plausible
that the latter cytokine release may be
responsible for the neutrophilic infiltration
observed in cases of herpetic keratitis.
Immunoglobulins
The three isotypes detected in the cornea are
IgM, G and A. IgG predominates in the central
cornea37. By contrast, IgM predominates in the
limbal region. The latter antibody is restricted
from the central cornea owing to its large size.
Antibodies in the cornea are found in the
stroma since they are cationically charged. Due
to their positive ionic charge, they bind to
proteoglycans and the anionic
glycosaminoglycans38.
Antigen/antibody complexes may sometimes
be observed in the corneal stroma in a variety
of pathological conditions (e.g. herpes simplex
keratitis), and because of their ringed shaped
appearance are known as “Wessley rings”.
Complement
The elements of complement found in the
cornea include C1-739 in addition to the
regulatory proteins H and I and C1 inhibitor40.
Interestingly, C1 is present in high
concentrations in the limbus and is restricted
from the central cornea. This finding is
significant since the limbal region is susceptible
to ulceration following complement activation
and immune complex deposition. It has been
suggested that C1 is produced by corneal
fibroblasts whereas the remaining components
detected in the cornea appear to be derived
from the plasma via linked vessels.
Conjunctival immunology
The conjunctival associated lymphoid tissue
(CALT) is part of the more general mucosa
associated lymphoid tissue (MALT). Langerhans’
cells and lymphocytes exist within the
conjunctival epithelial layer. In the substantia
propria, neutrophils, lymphocytes, IgA and IgG,
dendritic cells and mast cells all reside. It is
noteworthy that eosinophils and basophils are
not present in the healthy conjunctiva.
Langerhans’ cells and dendritic cells present
the antigenic peptide to the conjunctival T
helper cells. Following antigenic presentation,
the T cells secrete the cytokine IFN-γ which
serves to promote antigen elimination by
macrophages. This is the delayed-type
hypersensitivity (DTH) response (see later) and
is characteristic of conjunctival pathology such
as phlyctenulosis.
Scleral immunology
There exists only a small number of immune
cells in the sclera compared to the conjunctiva
since it is relatively avascular. In its resting
state, IgG appears to be present in large
amounts41. However, a sclera under stress, may
become immunologically active as a result of
migrating cells from the overlying episcleral and
underlying choroidal vasculature42,43.
Uveal immunology
The uveal tract is an important site from an
immunological perspective for two reasons:
first, it is highly vascular in nature; and second,
the majority of vessels are highly fenestrated
which facilitates the recruitment of leukocytes
during the inflammatory cascade. The uvea
contains numerous cellular components of the
immune system including macrophages, mast
cells, lymphocytes and plasma cells in addition
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32 www.optometry.co.uk
to an appreciable number of immune factors.
Although IgG and IgM have been detected in
both the choroid and iris, they exist in greater
numbers in the former structure. The reason for
such disparate levels is the dearth of anionic
antibody binding sites on the iris surface44. By
contrast, the ciliary body harbours tremendous
amounts of IgG by virtue of its anionic tissue
sites.
Immunopathology and the eye
Immunopathology encompasses both pathology
as a result of an over-active immune system
(hypersensitivity and autoimmunity) together
with that acquired through an individuals
inability to fight off infection – namely
immunodeficiency. Unfortunately, it is far
beyond the scope of this article to describe the
latter and its ophthalmic correlates.
In this section, the classification system
pertaining to hypersensitivity reactions will be
described and each subtype with an anterior
segment manifestation will be discussed. The
association between HLA and systemic and
ophthalmic disease will be illustrated together
with a brief overview of the concepts pertaining
to autoimmunity.
Hypersensitivity
The term hypersensitivity refers to the process
whereby the adaptive immune response overreacts
to a variety of infectious and inert
antigens resulting in damage to the host tissue.
Five types of hypersensitivity reactions exist –
all of which vary in their timing following
contact with the antigen (Table
.• Type I hypersensitivity: allergy
Allergies may affect approximately 17% of the
population45. The term atopic is used to describe
those individuals who possess a genetic
predisposition to allergy. Allergies may occur to
otherwise innocuous antigens (known as
allergens) and infectious agents, e.g. worms.
Type I hypersensitivity exists in two phases, the
sensitisation and effector phases.
Firstly, a harmless allergen causes production
of IgE antibody on first exposure. This IgE
February 8, 2002 OT
diffuses throughout the body until it comes into
contact with mast cells and basophils. Both
these cell types have receptors for IgE antibody.
Although the patient experiences no symptoms
after the initial binding, reintroduction of the
antigen/allergen induces the production of
more IgE and, furthermore, increase the
likelihood of cross-linking with existing
antibodies on the mast cell surface. Such crosslinking
induces the mast cell to degranulate and
release a host of inflammatory mediators such
as histamine, prostaglandins and bradykinin.
Histamine characteristically causes the itchy
symptoms experienced by patients and as a
result of binding to H1 receptors in the eye,
induces vasodilation and enhances mucous
secretion by the goblet cells. Bradykinin
augments vascular permeability, decreases
blood pressure and contracts smooth muscle.
Prostaglandins are also powerful inflammatory
mediators.
Ocular correlates:
The following anterior segment conditions are
due, if not only in part, to type I
hypersensitivity:
• Seasonal allergic conjunctivitis (type I)
• Giant papillary conjunctivitis (types I and IV)
• Vernal keratoconjunctivitis (VKC)
(type I and IV)
• Atopic keratoconjunctivitis (AKC)
(types I and IV) (Figure 7)
Seasonal allergic conjunctivitis can be easily
recognised by chemosis and hyperaemia of the
conjunctiva, lid swelling and excessive
lacrimation. The cornea is not affected.
GPC is well known to optometrists as an
allergic response to contact lenses, prosthetic
lenses, protruding corneal sutures and scleral
buckles. Histological examination of eyes
suffering from GPC have revealed degranulated
mast cells (type I hypersensitivity)46 and CD4+ T
cells (type IV)47, thus corroborating the theory
that such a condition is mediated by both types
of hypersensitivity.
Vernal conjunctivitis can present either in a
palpebral form characteristically exhibiting
giant cobblestone papillae, or in a limbal form
with gelatinous deposits known as Trantas’-dots,
which represent degenerating epithelial cells and
eosinophils. The first corneal change is a
punctate epithelial keratitis, which if left
unchecked develops into a macroerosion
(Figure
and finally a corneal plaque develops(Figure 9). The conjunctiva itself is
characteristically oedematous and contains an
array of immunological cells including
lymphocytes and mast cells. Evidence of type I
involvement includes the histological detection
of, inter alia, degranulated mast cells,
eosinophils and increased levels of IgE in
affected eyes49. The detection of CD4+ T cells and
macrophages is indicative of a delayed
inflammatory component49.
Table 8:
Hypersensitivity reactions
Hypersensitivity Appearance Mediators Mechanism
type time
I. Immediate 2-30 mins IgE Mast cell response
(enhance acute inflammation)
II. Cytotoxic 5-8 hours IgM and IgG Antibody and complement
III. Immune 2-8 hours IgM and IgG Antibody/antigen
complex immune complexes complexes
IV. Delayed type 24-72 hours CD4 and CD8 T cells, T cell mediated
APCs Macrophages activated
Include granulomatous reactions
V. Stimulatory Autoantibodies Autoantibodies against hormone
Figure 8:
Epithelial macroerosion in VKC
Figure 9:
Corneal plaque in VKC
Figure 7:
Corneal thinning and vascularisation in AKC,
excess mucous is present
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Module 4 Part 2 a
Type II hypersensitivity: cytotoxic
This classification of hypersensitivity involves
either IgG or IgM antibodies, which may induce
cellular lysis due to the involvement of the
classical complement pathway (as seen in blood
transfusion reactions) or recruit and activate
inflammatory cells via complement. The
components of complement include the C5a,
which serves to attract inflammatory cells to the
site of interest. The hypersensitivity reaction is a
result of the excessive amount of extracellular
mediators released by the inflammatory cells to
antigens that are too big to be completely
phagocytosed.
Antibodies to self-antigens, such as the
acetylcholine receptor in myasthenia gravis, is
not only another example of type II
hypersensitivity but also an example of
autoimmune disease.
Ocular manifestations
• Mooren’s ulcer
• Cicatrical pemphigoid
Mooren’s ulcer is a rare peripheral ulcerative
keratitis that exists either as a unilateral,
non-progressive form which has a predilection
for elderly patients or as a more severe,
progressive form affecting both eyes of relatively
young individuals. The signs range from a small
patch of grey infiltrate near the margin to frank
ulceration involving the entire corneal
circumference and, in some cases, the central
region as well. The healing process results in a
thin, vascularised, opaque cornea. Investigators
have identified a significant number of
lymphocytes, neutrophils and plasma cells50 in
the cornea, thus providing unequivocal evidence
to support the theory that this condition has an
immunopathological aetiology.
Cicatrical pemphigoid is a chronic, blistering
disease, which has a predilection for both the
ocular and oral mucous membranes. Unlike its
self-limiting counterpart, pemphigus vulgaris,
cicatrical pemphigoid rarely affects the skin.
Ocular cicatrical pemphigoid is a serious,
bilateral condition that represents effective
shrinkage of the conjunctiva. Although the
initial presentation may be subacute and
non-specific, it frequently progresses to
symblepharon, entropion with secondary
trichiasis, dry eye, ankyloblepharon and
conjunctival fornix shortening. There is evidence
to support the presence of IgG antibodies
directed against self-antigen in the basement
membrane of both the skin and eye51. Binding of
the aforementioned antibodies may activate
complement with subsequent recruitment of
inflammatory cells into the area. The process of
cicatrisation is achieved through the secretion of
collagen via fibroblasts as a result of stimulation
via cytokines released from the invading
inflammatory cells.
Type III hypersensitivity:
immune complex
Large pathogens with multiple antigenic sites
have several antibodies bound to them forming
immune complexes. Normally, these complexes
are removed by the mononuclear phagocytes in
the liver and spleen with no adverse sequelae.
However, persistence of immune complexes does
occur in certain individuals leading to their
deposition in tissue. As a consequence of the
latter action, complement may be activated thus
paving the way for inflammatory cells to enter
the deposition site. Since blood vessels (which
filter plasma at high pressure and exhibit a great
deal of tortuosity) are more susceptible for
immune complex deposition, the ciliary body is
particularly vulnerable to this type of
hypersensitivity reaction.
Ocular manifestations
• Uveitis (Crohn’s disease)
• Peripheral corneal lesions associated with
rheumatoid arthritis
• Stevens Johnson syndrome
• Sjögren’s syndrome
The signs and symptoms of the above will be
described in future articles in the series.
Type IV hypersensitivity: delayed-type
hypersensitivity (DTH)
The term DTH has been used to describe such a
reaction owing to its prolonged time-scale
relative to the other hypersensitivity types.
Although DTH can be transferred by T cells that
have been previously sensitised by an antigen, it
cannot be transferred in serum.
The sequence of events leading to DTH begins
with initial presentation of the antigen peptide
to T cells by APCs (e.g. Langerhans’ cells). The
primed T cell migrates to the site of antigenic
entry whereby it releases pro-inflammatory
mediators such as TNF. The release of these
cytokines facilitates blood flow and extravasation
of plasma contents to the area. The activation of
CD4 T helper and CD8 cells results in the release
of IFN-γ and, as a consequence, enhances
macrophage activity in that area. Resolution of
DTH is dependent on the efficacy with which
such phagocytes can remove the offending
antigen.
Recalcitrant infectious agents result in a
chronic DTH that causes the chronically activated
macrophages to fuse together and form
multinucleated giant cells. In an attempt to
contain the infectious agent, macrophages may
undergo further inter connections to resemble an
epithelial layer. Owing to the similarity to this
layer they are referred to as epitheloid cells.
Both epitheloid and multinucleated giant cells
secrete factors that induce fibrosis resulting in
granuloma formation. Thus granulomata are the
hallmark of chronic inflammation. Damage and
loss of function of the neighbouring tissues
frequently ensues until the agent is removed
either chemically or surgically.
Ocular manifestations
• Ocular allergies (VKC, AKC GPC)
• Idiopathic uveitis
• Sympathetic ophthalmia
• Phlyctenulosis
Type V hypersensitivity
This relatively new category encompasses the
concept of autoantibodies binding to hormone
receptors that mimic the hormone itself. This
results in stimulation of the target cells.
Examples include thyrotoxicosis.
Autoimmunity
The ability to react against self-antigens is
known as autoimmunity. However, a significant
number of people exist who harbour autoantibodies
and yet remain asymptomatic. The
corollary of this is that the presence of
autoreactive cells per se is not sufficient to
trigger autoimmune disease. In fact,
autoimmune disease is a result of breakdown of
one of the immunoregulatory mechanisms.
Furthermore, autoimmune disease may be
classified as either being organ specific (e.g.
insulin dependent diabetes mellitus, Grave’s
disease) or non-organ specific (e.g. Sjögren’s
syndrome, ankylosing spondylitis).
It is important to realise that the causes of
autoimmune diseases are multifactorial. The
main predisposing factors are age, gender,
infection and genetics. However, one of the
most important factors of interest to
immunologists and clinicians alike is the
association between HLA and autoimmune
disease. When determining the likelihood of
contracting a disease both epidemiologists and
clinicians refer to the relative risk. In the case of
HLA antigen, the relative risk compares the
chance of a person who has a particular HLA
antigen acquiring a disease to those individuals
who do not have such an antigen. The
association is exemplified by the relative risk of
suffering from ankylosing spondylitis (AS) in
individuals who possess HLA-B27. In patients
suffering from AS, the prevalence of HLA-B27 is
90% and this figure rises to 95% in patients
who suffer both with the disease and acute
iritis. Indeed, in the UK approximately 45% of
patients who present with acute iritis will
harbour HLA-B2752.
It is therefore important that patients who
present with anterior uveitis are screened for
HLA-B27 because although a positive result may
not necessarily be diagnostic, its presence will
certainly improve the sensitivity of further
radiological tests.
Table 9 compares the HLA associations with
both ophthalmic disorders together with those
systemic disorders relevant to the ophthalmic
practitioner.
Conclusion
This article has only broached the fascinating
subject of ocular immunology. A basic
understanding of immunology is required if
practitioners are to therapeutically manage their
patients. Further articles in the series will help
to reinforce the concepts of this challenging
subject.
Acknowledgement
The author would like to thank Professor Roger
Buckley for his permission to use Figures 7 to 9.
ot
34 February 8, 2002 OT www.optometry.co.uk
Figures 1 to 6 reprinted from
Immunology, Third Edition, Roitt,
Brostoff and Male, 1993 by
permission of the publisher
Mosby.
About the author
Gregory Heath is an optometrist
working part-time in private
practice. He was recently awarded
the diploma in clinical optometry
at City University and is currently
reading medicine at the Royal
Free and University College,
London, Medical School.
References
References are available upon
request. Please fax 01252-816176
or email info@optometry.co.uk.
Table 9:
HLA and ophthalmic disease
Disease HLA Relative risk 54 Ocular
association manifestation
Ankylosing spondylitis B27 90 Anterior uveitis
Reiter’s disease B27 33 Mucopurulent conjunctivitis
Anterior uveitis
Keratitis
Rheumatoid DR4 7 Keratoconjunctivitis sicca
Keratitis
Scleritis
Primary Sjögrens’ syndrome DR3, DR5 10 Keratoconjunctivitis sicca
Sarcoidosis DR3 Not known Anterior uveitis (acute and chronic)
Dacryoadentis
Retinal vasculitis,
neovascularisation,
Optic nerve granulomata
Cicatrical pemphigoid DQw7 Not known Shrinkage of conjunctiva
Behcet’s disease B5 3 Anterior uveitis
Retinitis, periphlebitis,
retinal oedema
Sympathetic DR4, A11, Not known Panuveitis
ophthalmia B40
Systemic lupus DR2, DR3 3 Punctate epithelial keratopathy
erythematosus Keratopathy
Necrotising scleritis
Retinal lesions (cotton wool spots)
Autoimmune optic neuropathy
1. Which one of the following is not part of the
innate immune system?
a. Mast cells
b. Complement
c. Phagocytes
d. T cells
2. Which one of the following statements is
correct regarding the innate immune system?
a. It is specific
b. It evokes a more potent response on
secondary exposure
c. It represents the first line of defence
d. It is able to memorise pathogens on
subsequent exposures
3. Which one of the following statements is
correct regarding the cells of the innate
system?
a. Basophils are important phagocytes
b. During phagocytosis the pathogen becomes
initially internalised as a phago-lysosome
c. Eosinophils play an important role in
combating virally-infected cells
d. Langerhans’ cells form a bridge between
innate and adaptive immunity
4. Which one of the following statements is
correct regarding the adaptive immune
system?
a. It consists of all types of lymphocytes
b. T cells produce antibodies
c. T cells maturate in the thymus
d. B cells are produced in the spleen
5. Which one of the following statements is
correct regarding T cells?
a. T cells can be subdivided into TH1 and TH2
subtypes only
b. T cells alone can identify any type of antigen
c. T cells express cell surface proteins denoted by
cluster determinant (CD) numbers
d. All T cells are involved in initiating the
inflammatory response
6. Which one of the following statements is
incorrect?
a. B cells have antibodies as their cell surface
receptor
b. There are five types of antibody
c. IgE is an important antibody in allergies
d. All B cells differentiate into plasma cells
7. Which one of the following statements is
correct regarding ocular immune privilege?
a. There is an absence of Langerhans’ cells in the
central cornea
b. Aqueous humour has no role
c. Abundant vascular supply is vital
d. All ocular cells express MHC class II
8. Which one of the following statements
concerning ocular immunology is incorrect?
a. Levels of IgA and complement
increase when the eyes are closed
b. IgA is the predominant antibody
in blood and tissue fluid
c. IgM, IgG and IgA antibody isotypes have been
identified in the cornea
d. The sclera contains a smaller number of
immune cells than the conjunctiva
9. In a patient suffering from vernal conjunctivitis,
which one of the following statements is
correct?
a. Trantas’-dots represent infiltrating T cells
b. Affected eyes have increased levels of IgD
c. Histologically, mast cells, lymphocytes and
macrophages have been identified
d. Is due to type III Hypersensitivity
10. Which one of the following statements is
incorrect?
a. HLA-B27 is a risk factor for both anterior uveitis
and ankylosing spondylitis
b. Granulomas are present in type IV
hypersensitivity reactions
c. Histamine is an important vasoconstrictor
d. IgE mediated hypersensitivity is of rapid onset
11. What proportion of patients with acute iritis will
harbour HLA-B27?
a. 15%
b. 25%
c. 45%
d. 65%
12. Which one of the following statements is correct
regarding a type I hypersensitivity reaction?
a. It always occurs in isolation
b. It is characterised by the presence of
macrophages
c. It is associated with myasthenia gravis
d. It involves the degranulation of mast cells
following the cross-linking of IgE bound to its
cell surface
Multiple choice questions - Basic immunology Please note there is only one correct answer
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