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19
2
Antigens and
Antibodies
OBJECTIVES
At the end of this chapter you should be able to:
• Understand what makes one substance more
antigenic than another.
• Define ‘epitope’, ‘immunodominant’ and
‘hapten’.
• Discuss the role of adjuvant in the immune
response.
INTRODUCTION
This chapter reviews two very basic elements of the
immune response (antigens and antibodies) and the
means by which these molecules interact in vivo and
in vitro. Understanding of the core concepts presented
here underpins much of the remainder of the material
covered in this book.
ANTIGENS
An antigen may be simply defined as a substance that
binds to a lymphocyte receptor. In associating with
the receptor the antigen may or may not initiate an
immune response. Classically, an antigenic molecule is
defined by its ability to be bound by a specific
antibody (see below), but some antigens fail to
stimulate antibody production as part of the immune
response. The related term immunogen refers to a
substance that induces an immune response when
injected into an individual. Antigens may be one of
several diverse classes of molecules that interact with
the immune system. The majority of antigens are
foreign to the body and may enter a host via a range
of possible routes (e.g. percutaneous absorption,
injection, ingestion, inhalation, sexual contact). These
• Describe the basic structure of the
immunoglobulin molecule.
• List the five classes of immunoglobulin and
describe the structure and function of each.
• Describe how an antigenic epitope binds to an
antibody.
• Define ‘affinity’ and ‘avidity’ as they relate to
antigen binding by antibody.
are referred to as heteroantigens and include
infectious agents (e.g. viruses, bacteria, fungi,
protozoa, helminths), environmental substances (e.g.
pollens, pollutants) and chemicals (e.g. drugs). Other
classes of antigen arise from tissue or cells. These
include alloantigens carried by foreign cells or tissue
from a genetically dissimilar member of the same
species that is grafted into an individual (i.e. during
transplantation), or xenoantigens present on graft
tissue derived from a different species (e.g. the
transplantation of porcine heart valves into a human).
Finally, in some circumstances it is possible for the
immune system to recognize components of the host’s
own body (autoantigens or ‘self-antigens’), a situation
that might give rise to autoimmune disease.
There are certain properties of an antigen
that determine the potency of the immune response
made to that antigen. This is referred to as the
antigenicity of the substance. The most effective
antigens:
• Are foreign to the host.
• Are of molecular mass >10 kD.
• Are particulate or aggregated (small soluble
molecules are poorly antigenic).
• Have a complex (tertiary) molecular structure.
• Carry a charge.
• Are chemically complex.
• Are biologically active.
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VETERINARY IMMUNOLOGY
The latter property means that infectious agents are
generally excellent antigens, although some organisms
have developed the ability to evade the host immune
response and induce chronic persistent infection (see
Chapter 1, p. ??).
Given that the majority of antigens are
structurally complex, there are often numerous
distinct regions within the antigen that are
individually capable of interacting with the immune
system. One antigen may therefore comprise multiple
epitopes or determinants, each of which may be
bound by a specific antibody or cellular receptor
molecule (2.1a–d). Some epitopes within an antigen
may be more effective at inducing an immune
response (as they incorporate more of the properties
listed above), and these are known as the
immunodominant epitopes.
2.1
A discussion of antigens must include mention of
a hapten, defined as a small chemical group which, by
itself, cannot elicit an immune response, but when
bound to a larger carrier protein is capable of
generating an antibody or cellular immune response
(2.2). The clinical relevance of this phenomenon lies
in the fact that some drugs may bind carrier proteins
and inappropriately stimulate a drug reaction (2.3).
The science of immunology most often involves
the study of the interaction of antigens with the
immune system. These investigations are often
performed experimentally by exposing an animal to
an antigen and documenting the ensuing immune
response. In such an experimental context we have
learnt that the potency of the immune response
generated can be determined by a number of factors.
These lessons have direct clinical relevance where an
antibody
epitope
antigen
peptide
fragment
T
(a)
(c)
(b)
T-cell receptor
(d)
2.1a–d Antigenic epitopes. (2.1a) A complex antigen may be composed of numerous individual epitopes each capable of interacting with the
host immune system. The most potent of these epitopes are considered immunodominant. (2.1b, 2.1c) This German Shepherd Dog has
staphylococcal infection of the hair follicles (deep pyoderma) and colonies of staphylococci are seen within the follicular lumen on biopsy. (2.1d)
Each individual Staphylococcus may be considered an antigen composed of multiple epitopes capable of inducing an antibody response. In this
experiment, staphylococci have been disrupted and the component epitopes separated by molecular mass through polyacrylamide gel
electrophoresis. The epitopes have been electophoretically transferred to an inert membrane and demonstrated by use of an antiserum in the
technique of western blotting.
21
ANTIGENS AND ANTIBODIES
2.3
2.2
antibody to
hapten
antibody to
hapten-protein
antigen
(a)
OH
antibody to
protein (native
epitope)
N
N
CH2
C
CI
N
2.2 Hapten. This small chemical group is nonimmunogenic unless it
is conjugated to a large carrier protein. The combined hapten–carrier
may trigger an immune response (represented by antibody) to the
hapten alone, the carrier protein alone, or a novel antigen formed of
both the hapten and carrier.
R
(b)
CI
2.3a, b Drug reaction. (2.3a) This German Shepherd Dog was being
treated with the systemic antifungal drug ketoconazole (chemical
formula shown) and subsequently developed lesions affecting the
planum nasal and periorbital skin. (2.3b) In this instance the drug
may be acting as a hapten by binding to dermal protein and
triggering a local immune response. Such reactions may
spontaneously resolve once administration of the drug is halted.
immune response is artificially induced in an animal
through vaccination. The factors that determine the
efficacy of an immune response include:
• The method of preparing the antigen.
• The species and genetic background of the
recipient.
• The dose of antigen administered.
• The route of administration.
• The use of an adjuvant.
An adjuvant is a substance which, when combined
with antigen, nonspecifically enhances the ensuing
immune response to that antigen. Adjuvants
additionally lead to general enhancement of immune
function. Adjuvants such as Freund’s adjuvant (an oil
in water emulsion), Freund’s complete adjuvant
(incorporating killed mycobacteria into the emulsion)
or peanut oil are highly irritant substances that induce
a local inflammatory response and stimulate antigen
presentation (see Chapter 7, p. ??). Adjuvants such as
these may also produce a depot effect, whereby small
quantities of antigen are slowly released from the
antigen–adjuvant amalgam to maintain a more
prolonged antigenic stimulation. Many of the vaccines
currently used in veterinary and human medicine are
adjuvanted to increase the immunogenicity of the
antigenic component of the vaccine (see Chapter 20,
p. ??). Vaccine adjuvants must be considerably safer
than the selection described above and the most
commonly employed is alum (aluminium hydroxide
or aluminium phosphate). Another form of adjuvant
is the liposome or iscom (immune stimulatory
complex) in which antigen is held within a minute
vesicle made up of lipid membrane. The search for
more effective and safer adjuvants is an active area of
research. The latest generation of adjuvants may have
a more targeted effect, enhancing particular aspects of
immunity. Small fragments of bacterial DNA rich in
cytosine and guanidine (CpG motifs) or recombinant
immune-modulatory cytokines (see Chapter 7, p. ??
and Chapter 20, p. ??) are such substances. They are
collectively are referred to as molecular adjuvants.
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VETERINARY IMMUNOLOGY
ANTIBODIES
The fraction of blood that remains fluid after
clotting represents the serum (i.e. plasma without
fibrinogen). It may be electrophoretically separated
into constituent proteins including albumin and the
alpha, beta and gamma globulins (2.4). The gamma
globulins comprise chiefly the immune globulins
(immunoglobulins) also known as antibodies.
The basic molecular structure of a single
immunoglobulin molecule is well characterized (2.5).
It is comprised of four glycoslyated protein chains
held together by inter-chain disulphide bonds in a Yshaped conformation. Two of the chains are of higher
molecular mass (heavy chains, approximately 50kD)
and two chains are smaller in size (light chains,
approximately 25 kD). In terms of structure and
amino acid sequence, the two heavy chains in any one
immunoglobulin are identical to each other, as are the
two light chains. This means that the two ‘halves’ of
the molecule are essentially mirror images of each
other.
Although
we
commonly
depict
immunoglobulins diagrammatically as linear
structures, these molecules have a complex tertiary
structure. Each chain is formed of a series of domains
that have a roughly globular structure that is created
by the presence of intrachain disulphide bonds. A
further basic feature of an immunoglobulin is that the
structure and amino acid sequence towards the Cterminal end of the molecule is relatively uniform
(conserved) from one molecule to another, while the
structure towards the N-terminal end has
considerable variation between immunoglobulins.
Within each of the four chains this gives rise to the
presence of an N-terminal variable region and a series
of constant regions towards the C terminal.
Therefore, each light chain is composed of two
domains, a variable region of the light chain VL and a
constant region of the light chain CL. Similarly, each
heavy chain is comprised of a variable region of the
heavy chain VH and a series of constant region
domains named CH1, CH2 and CH3. The variable
regions contain further subareas in which there is the
greatest degree of variation in amino acid sequence
between different immunoglobulins. These subareas
are known as the hypervariable regions. Most
immunoglobulins have a distinct region between the
CH1 and CH2 domains, involving an interchain
disulphide bond known as the hinge region. This
confers on the molecule the ability of the short arms
of the Y-shaped structure to move through
approximately 180o.
The globular domains of the immunoglobulin
molecule have distinct functional attributes. The
structure formed by the variable regions (VL and VH)
is that portion of the molecule that binds to an
antigenic epitope (the antigen-binding site). The CH2
domains are involved in activation of the complement
pathway (see Chapter 3, p. ??) and the CH3 domains
in binding of the molecule to cellular immunoglobulin
receptors called Fc receptors (see Chapter 3, p. ??).
Early immunological research characterized the
way in which the Y-shaped immunoglobulin molecule
may be fragmented by incubation with particular
proteolytic enzymes (2.5). The enzyme papain cleaves
the molecule on the N-terminal side of the hinge
region, creating three fragments: the joined Cterminal heavy chains (the ‘body’ of the Y shape),
known as the Fc region; (for ‘fragment crystallizable’);
and the joined N-terminal heavy chains and light
chains (the ‘arms’ of the Y shape), each known as an
Fab fragment (‘fragment antigen-binding’). In
contrast, the enzyme pepsin cleaves the molecule on
the C-terminal side of the hinge region, creating two
fragments: an Fc region and both Fab fragments still
joined through the intact hinge region disulphide
bond (the Fab’2).
IMMUNOGLOBULIN CLASSES
2.4
α
β
β globulins
albumin
2.4 Serum protein electrophoresis. The constituent proteins of
serum include albumin and the alpha, beta and gamma globulins. The
latter are equivalent to antibody molecules or immunoglobulins.
At the simplest level, there are five distinct forms of
immunoglobulin heavy chain named by the Greek
letters α (alpha), δ (delta), ε (epsilon), γ (gamma) and
μ (mu). Similarly, there are two distinct forms of light
chain named κ (kappa) and λ (lambda). As will be
discussed in Chapter 9, this situation arises from the
fact that there is a series of genes that encode these
distinct protein molecules. Given that one Y-shaped
immunoglobulin is composed of two identical heavy
and two identical light chains, a single
immunoglobulin must therefore consist of a pair of
one of the five types of heavy chains, coupled to a pair
of either κ or λ light chains. In this way, there are ten
possible ways in which these chains may combine to
form an antibody molecule. Five immunoglobulin (Ig)
classes are defined by usage of these heavy chain
23
ANTIGENS AND ANTIBODIES
2.5 Immunoglobulin structure. The basic Y-shaped structure of an
immunoglobulin is composed of two identical heavy chains and two
identical light chains linked together by interchain disulphide bonds.
Intrachain disulphide bonds confer a globular domain structure to
each chain. The N-terminal domains (VL and VH) show greatest
variation in amino acid sequence between immunoglobulins and
represent the site of antigen binding, particularly through the
hypervariable regions. The C-terminal domains (VH, CH1 to CH3) have
conserved amino acid sequence and undertake functions including
complement fixation and receptor binding. The hinge region between
CH1 and CH2 allows mobility of the short arms of the molecule. The
enzyme papain cleaves the molecule on the N-terminal side of the
hinge, creating a single Fc and two Fab fragments. The enzyme
pepsin cleaves the molecule on the C-terminal side of the hinge,
creating one Fc and one Fab’2 fragment.
2.5
hypervariable
regions
antigenbinding
site
VH
CH1
VL
CL
hinge region
CH2
C H3
molecules: IgA (α chain), IgD (δ chain), IgE (ε chain),
IgG (γ chain) and IgM (μ chain). For IgG and IgA,
further diversity amongst the genes encoding the
heavy chains means that there are subclasses of
immunoglobulin that have subtle differences between
the amino acid sequence and structure of the constant
regions.
The best characterized immunoglobulins and
immunoglobulin genes are those of humans and
experimental rodents. However, readers should be
aware that while extrapolations may be made to
domestic animal species, distinct species differences
exist. Domestic animal species share the same basic
five immunoglobulin classes, but there is variation in
the range of IgG and IgA subclasses and some unique
species-specific modifications in immunoglobulin
structure exist (Table 2.1 overleaf ). For example, four
IgG subclasses (IgG1–IgG4) are recognized in humans
and dogs, while there are seven in the horse
(IgG1–IgG7). Humans have two IgA subclasses
encoded by distinct IgA heavy chain (IGHA) genes
and rabbits have 13 such genes, most of which
produce functional protein. In contrast, domestic
animals have a single IGHA gene, but genetic
variation in the hinge region of the IgA heavy chain
gene has been identified in pigs, sheep and dogs,
which may translate into functionally distinct IgA
proteins. In addition to standard four-chain IgG,
camelid species produce homodimeric IgG made up of
two heavy chains and no light chain. Chickens have a
major immunoglobulin known as IgY, a four-chain
molecule not dissimilar to mammalian IgG in
structure. The heavy chain of IgY (υ or upsilon chain)
has one variable and four constant domains, but lacks
a hinge region. Some avian species also have a
truncated version of IgY with heavy chains of only
two constant domains. As this molecule does not have
an Fc region it is known as IgY(ΔFc) and is of
unknown function.
IgG
The IgG molecule comprises a single Y-shaped unit
with a molecular structure as described above. IgG
consists of paired γ heavy chains with either paired κ
or λ light chains. The molecule has two N-terminal
antigen-binding sites and is therefore said to have an
antigen-binding valence of two. IgG is the dominant
form of immunoglobulin found in the serum. Where
IgG subclasses are defined, the relative serum
concentrations of these may vary. The molecular mass
of IgG (approximately 150 kD) means that it is
readily able to leave the circulation and enter the
extracellular tissue space when there is increased
permeability of the vascular endothelium (e.g. in an
inflammatory response), but some IgG is normally
found in this environment and may be actively
transported there from the blood. The functional
properties of IgG include complement fixation,
immunoglobulin Fc receptor binding and
opsonization (see Chapter 3, p. ??). IgG molecules
effectively bind and neutralize toxins and are the
dominant immunoglobulin in the secondary immune
response (see Chapter 9, p. ??).
24
VETERINARY IMMUNOLOGY
Table 2.1. IgG, IgA and IgM in man and animals.
Species
IgG subclasses
Serum IgG
concentration
(mg/ml)
IgA subclasses1
Serum IgA
concentration
(mg/ml)
Serum IgM
concentration2
(mg/ml)
Man
G1, G2, G3, G4
7.5–22.0
A1, A2
0.5–3.4
0.2–2.8
Dog
G1, G2, G3, G4
10.0–20.0
0.2–1.6
1.0–2.0
Cat
G1, G2, G3
5.0–20.0
Not recognized
0.3–2.0
0.2–1.5
G1, G2, G3, G4,
11.5–21.0
Not recognized
1.0–4.0
1.0–3.0
Horse
Four allelic variants
in hinge region
G5, G6, G7
Cow
G1, G2, G3
17.0–27.0
Not recognized
0.1–0.5
2.5–4.0
Sheep
G1, G2, G3
18.0–24.0
Three allelic variants
0.1–1.0
0.8–1.8
G1, G2a, G2b, G3, G4
9.0–24.0
Two allelic variants
0.5–1.2
1.9–3.9
1.0–3.2
0.8–6.5
in hinge region
Pig
in hinge region
Mouse
G1, G2a, G2b, G3
2.0–5.0
Two allelic variants
in hinge region
1 Two
IGHA genes are recognized in man, but only one in other species.
2 Subclasses
of IgM are not recognized.
IgM
The IgM molecule is the largest of the
immunoglobulins and consists of a pentamer of five
basic Y-shaped immunoglobulin units linked together
by a joining (J) chain and additional disulphide bonds
between the C-terminal domains (2.6). Each of the
component units comprises paired μ heavy chains and
each of these carries an additional C-terminal CH4
domain. The IgM molecule lacks a distinct hinge
region, but there is mobility at the level of CH2 to CH3.
The large molecular mass of IgM means that this
immunoglobulin is mainly found in the circulation as
it would not readily diffuse between vascular
endothelia to enter the tissue space. For this reason,
IgM plays an important role in the immune response
to infections of the blood (e.g. bacteraemia). IgM is
able to fix complement (see Chapter 3, p. ??) and
because of its antigen binding valence of 10, it is able
to attach to and draw together multiple particulate
antigens in a process known as agglutination (2.7a,
b). IgM is of greatest importance in the primary
immune response (see Chapter 9, p. ??).
2.6
Cμ1
VH
VL
CL
Cμ2
Cμ3
Cμ4
2.6 Structure of IgM. IgM is a pentamer of five basic
immunoglobulin units linked together by a joining chain and
additional disulphide bonds between the CH3 and additional CH4
domains present in the μ heavy chain.
25
ANTIGENS AND ANTIBODIES
2.7
(b)
(a)
2.7a, b Agglutination by IgM. The size and antigen-binding valence (10) of IgM makes this molecule particularly effective at binding and
drawing together particulate antigens in the process of agglutination. A clinical example of this phenomenon occurs in immune-mediated
haemolytic anaemia (IMHA) in which IgM autoantibodies cause aggregation and destruction of erythrocytes within the circulating blood. (2.7a)
IgM molecules bind to different erythrocytes to form an agglutinate. (2.7b) A drop of blood from a dog with IMHA shows the presence of large
agglutinates of red blood cells.
2.8 Structure of IgA. Dimeric
IgA comprises two Y-shaped
immunoglobulin units with
heavy chains linked by a J chain.
Where secreted onto mucosal
surfaces the molecule also
carries a secretory component
that wraps around the Cterminal ends of the
immunoglobulin units to protect
the IgA from proteolytic
degradation.
2.8
VL
CL
Cα2 Cα3
J chain
secretory component
IgA
The IgA molecule may be found as a monomer of a
single Y-shaped immunoglobulin (utilizing α heavy
chains) or as a dimer of two such units linked together
by a J chain (2.8). These variants have an antigenbinding valence of 2 and 4, respectively. The
occurrence of monomeric versus dimeric IgA is
dependent on the species and anatomical location.
Relatively small quantities of IgA may be found
within the circulation as a monomer in humans and as
a dimer in most domestic animal species. The highest
concentration of IgA is found in the secretions bathing
the mucosal surfaces of the body (i.e. the
gastrointestinal, respiratory and urogenital tracts, the
eye and the mammary gland) or in secretions related
to these surfaces (e.g. bile, tears, colostrum, milk).
This distribution reflects the fact that the majority of
IgA in the body is produced at mucosal surfaces and
plays a key role in the immune defence of those
surfaces, particularly in preventing colonization by
pathogenic bacteria by inhibiting receptor-mediated
26
VETERINARY IMMUNOLOGY
attachment of such organisms to the mucosal surface
(2.9a, b). The IgA molecule is also able to neutralize
toxins, but is a relatively weak opsonin (see Chapter
3, p. ??).
This secreted form of IgA is dimeric in all species
and in those species with distinct IgA subclasses there
may be regional preferences for the secretion of those
subclasses to mucosal surfaces. As mucosal surfaces
are rich in proteolytic enzymes, secreted dimeric IgA
has one further modification that protects it from
enzymatic degradation once secreted. This
modification is the presence of a secretory component,
which wraps around the C-terminal ends of the
subunits and is attached to the CH2 domains by
disulphide bonds. The pathway of mucosal IgA
secretion has been defined (2.10). As for all
immunoglobulins, the dimeric IgA molecule is
secreted by a plasma cell (see Chapter 5, p. ?? and
Chapter 9, p. ??) within the lamina propria underlying
the mucosal epithelium. This molecule is bound by the
polymeric immunoglobulin receptor (pIgR) on the
basolateral surfaces of the epithelial lining cell (in the
intestine the epithelial cells at the base of the crypts
express pIgR) and the complex of pIgR and IgA dimer
is internalized into the cytoplasm of the epithelial cell.
The complex translocates through the cytoplasm and
is re-expressed on the mucosal surface of the cell,
where the IgA is released from the surface, carrying a
portion of the pIgR that becomes the secretory
component.
IgD
The IgD molecule is a single Y-shaped
immunoglobulin unit comprising two δ heavy chains
and with an antigen-binding valence of 2. IgD has
very limited distribution in the body and is restricted
to being expressed on the surface of naïve B
lymphocytes (see Chapter 9, p. ??). There are
numerous species differences in the structure of this
immunoglobulin. Murine IgD has two constant region
domains and an extended hinge region, making it
susceptible to proteolysis. IgD of domestic animals,
man and primates has three constant region domains.
In most of these species (except the pig) the hinge
region is also long relative to that of other
immunoglobulin classes.
2.9
(a)
(b)
2.9a, b Function of IgA. (2.9a) This scanning electron microscopic image shows the normal finger-like villi of the
small intestine. (2.9b) A higher magnification showing intestine from a different animal with diarrhoea caused by
Escherichia coli infection. There is a carpet of rod-shaped bacteria over the mucosal surface. Attachment of such
pathogens to mucosae would normally be inhibited by secreted IgA that blocks receptor-mediated attachment. (Images
courtesy G. Hall)
27
ANTIGENS AND ANTIBODIES
2.10
2.10 Secretion of IgA. Most
IgA is produced at mucosal
surfaces and secreted across
the lining epithelium as a
dimeric molecule. During this
transition the secreted IgA
molecule acquires a protective
secretory component that is
formed from a portion of the
polymeric immunoglobulin
receptor that mediates its
transition across the epithelial
barrier.
IgA secretion
plgR
IgA dimer
with secretory
piece
IgA plasma
cell
enterocyte
IgE
The IgE molecule is a single Y-shaped
immunoglobulin unit composed of two ε heavy chains
and it has a valence of 2. Similar to IgM, the IgE
molecule has an additional CH4 domain and an
indistinct hinge region. IgE is found at a low
concentration free in the circulation of humans, but at
a proportionally higher concentration in the blood of
most domestic animal species. This is thought to
relate to the relatively greater level of endogenous
parasitism of domestic animals, as one of the major
beneficial roles of IgE is as a participant in the
immune response to endoparasites (see Chapter 13, p.
??). IgE may also be found attached to specific Fcε
receptors on the surface of tissue mast cells and
circulating basophils, and it is intrinsic to the
immunological phenomenon known as type I
hypersensitivity (see Chapter 12, p. ??). Related to this
mechanism is the key role of IgE in mediating a range
of allergic diseases (e.g. asthma, atopic dermatitis, flea
allergy dermatitis) of major significance in human and
animal populations (see Chapter 17, p. ??) (2.11).
2.11
2.11 IgE in allergic disease. This cat is receiving inhaled medication
through a purpose-designed delivery system for the treatment of
asthma. Asthma is an immune-mediated disease involving the
excessive production of IgE antibodies specific for environmental
antigens.
28
VETERINARY IMMUNOLOGY
ANTIGEN–ANTIBODY INTERACTION
The interaction between antigens and antibodies
specifically involves the recognition of an antigenic
epitope by the N-terminal variable (Fab) regions of
the antibody molecule. In three dimensions, the VL
and VH domains form a cleft or groove, lined by the
hypervariable regions of amino acid sequence, into
which the epitope slots or, alternatively, these domains
flatten out to form a planar area of interaction that
permits a larger area of contact with the antigen. This
interaction is exquisitely precise and there are specific
points of interaction (contact residues) between the
epitope and antigen-binding site (2.12a, b). Some
antigenic epitopes will have a perfect match for the
antigen binding site of immunoglobulin and interact
like a ‘lock and key’ to produce a high affinity
binding. Antibody affinity refers to the strength of
binding of one Fab of an antibody to one antigenic
epitope (2.13). In this situation the antigen is held
tightly in place by interactions involving the
formation of van der Waal’s forces, hydrogen bonds,
electrostatic forces and hydrophobic forces. Other
antigenic epitopes will interact with antigen-binding
sites with low affinity or display no interaction at all.
A second term used to describe the strength of
interaction between antigen and antibody is avidity.
Avidity refers to the strength of overall binding of the
two molecules, such that a low avidity interaction
might involve the binding of one Fab within an
immunoglobulin to one epitope on an antigen,
whereas an interaction of higher avidity would
involve multiple Fab-epitope binding (2.14).
2.12
light chain
hypervariable
region
light
antigen
contact residues
3
1
hypervariable
regions
4
3
17
6 2
14
5
15
16
7
10
11
9
12 8
epitope
4
5
2
1
16
6 7
15
8 14
13
9
10 12
11
13
heavy
(a)
heavy chain
(b)
2.12a, b Antigen-antibody binding. (2.12a) The antigenic epitope sits within a cleft or groove formed by the variable domains of the heavy and
light chains and lined by hypervariable regions that form the contact residues between the two molecules. (2.12b) Depicted more three
dimensionally, the red hypervariable regions of the yellow light and blue heavy chain variable regions provide contact residues with the
corresponding red epitope of the green antigen. The green antigen in this model would move through 90o to sit on top of the Fab region shown.
29
ANTIGENS AND ANTIBODIES
2.13
attractive
forces
2.14
repulsive
force
antibody
epitope
high affinity
low affinity
2.13 Antigen–antibody binding affinity. Affinity is the sum of
attractive and repulsive forces between the antigenic epitope and the
Fab antigen-binding site of the antibody. High and low affinity
interactions are demonstrated.
low
moderate
high
2.14 Antigen–antibody binding avidity. The interaction between an
antigen comprised of repeating units of an epitope and specific
antibodies is shown. The greater the number of interactions the
higher is the avidity of binding.
KEY POINTS
• Antigens initiate the immune response.
• Most antigens are foreign to the body
(heteroantigens).
• The most potent antigens are chemically
complex and biologically active.
• Antigens may have multiple epitopes, some of
which are immunodominant.
• Adjuvants may be mixed with antigen to
enhance nonspecifically the immune response.
• Antibodies are Y-shaped units composed of two
identical heavy chains and two identical light
chains.
• The chains of an antibody molecule have a
globular domain structure, with a distinct
function ascribed to each domain.
• Antibodies bind antigenic epitopes, activate
complement and bind cellular receptors.
• The five classes of antibody (IgG, IgM, IgA, IgD
and IgE) have a distinct structure, anatomical
distribution and function.