Antigen
Antibody Interaction And Immune Complexes
An antigen, by definition, stimulates the
production of antibody, which in
turn combines with the antigen. Both processes are based on complementarity (or
‘fit’) between two shapes – a small piece of the antigen (or determinant)
and the combining site of the antibody, a cleft formed largely by the
hypervariable regions of heavy and light chains (see Fig. 14). The closer the
fit between this site and the antigenic determinant, the stronger the
non-covalent forces (hydrophobic, electrostatic, etc., lower left) between them
and the higher the affinity (top left). When both combining sites can
interact with the same antigen (e.g. on a cell), the bond has a greatly
increased strength, which in this case is referred to as ‘avidity’ (see Fig.
14).
The ability of a particular
antibody to combine with one determinant rather than another is referred to as specificity.
The antibody repertoire of an animal, stored in its V genes and expanded
further by mutation (see Fig. 13), is expressed as the number of different
shapes towards which a complementary specific antibody molecule can be made,
and runs into millions.
What happens when antigen and
antibody combine depends on the circumstances. Sometimes antibody alone is
enough to neutralize the antigen.
This is the case for toxins (such as tetanus or diphtheria) or microorganisms such as viruses that need to
attach to cell-surface receptors in order to gain entry (the ability to block
entry is often called neutralization).
Usually, however, a secondary
interaction of the antibody molecule with another effector agent, such as
complement or phagocytic cells, is required to dispose of the antigen. The
importance of these secondary interactions is shown by the fact that deficiency
of complement or myeloid cells can be almost as serious as deficiency of
antibody itself (see Fig. 33).
The combination of antigen and
antibody is called an immune complex; this may be small (soluble) or
large (precipitating), depending on the nature and proportions of antigen and
antibody (top right). The usual fate of complexes is to be removed by
phagocytic cells, through the interaction of the Fc portion of the antibody
with complement and with cell-surface receptors (bottom centre and see Figs 6
and 9). However, in some cases complexes may persist in circulation and cause
inflammatory damage to organs (see Fig. 36) or inhibit useful immunity, e.g. to tumours or parasites.
Antigen – antibody interaction
The combining site of antibody is a
cleft roughly 3 × 1 × 1 nm (the size of five or six sugar units), although
there is evidence that antigens may bind to larger, or even separate, parts of
the variable region. Binding depends on a close three-dimensional fit, allowing
weak inter- molecular forces to overcome the normal repulsion between
molecules. Although binding between antigen and antibody involves only
non-covalent forces, and therefore is theoretically reversible, in practice the
high affinity of most antibodies means that they rarely become detached
from their targets before these are destroyed.
Van der Waal’s forces attract all molecules through their
electron clouds, but only act at extremely close range.
Hydrogen bonding (e.g. between NH2 and OH groups) is another
weak force.
Electrostatic attraction between parts of an antibody and
antigen molecule with a net opposite charge (e.g. a negatively charged carboxyl
group and a positively charged ammonium group) is sometimes quite strong.
Hydrophobic regions on antigen and antibody will tend to be
attracted in an aqueous environment; this is probably the strongest force
between them.
Affinity is normally expressed as the association
constant under equi- librium conditions. A value of 105 L/mol would be considered
low, while high-affinity antibody can reach 1010 L/mol and more, several orders
of magnitude higher than most enzyme – substrate interactions. In practice, it
is often avidity that is measured because antibodies have (at least) two
valencies, and even with monovalent antigens a serum can only be assigned an
average affinity. Average affinity tends to increase with time after antigenic
stimulation, partly through cell selection by diminishing amounts of antigen,
and partly via somatic mutation of Ig genes. High-affinity antibodies are more
effective in most cases, but low-affinity antibodies persist too, and may have
certain advantages (reusability, resistance to tolerance?).
Antigen It is remarkable that the process of making
antibodies is so versatile that one can find antibodies specific
for almost any type of molecular surface. These include
the common components of pathogens such as proteins, nucleic acids, sugars and
lipids, but also man-made synthetic molecules. For example, antibodies against amphetamines are being
developed for the treatment of drug addiction.
Under conditions of antigen or
antibody excess, small (‘soluble’) complexes tend to predominate, but with
roughly equivalent amounts of antigen and antibody, precipitates form, probably
by lattice formation. Such precipitates activate the inflammatory response and
probably underlie some types of occupational allergies, such as ‘Farmer’s
lung’ (see Figs 35 and 36). Complexes
between antibodies and large antigens
(e.g. nucleic acids) are associated with systemic autoimmune diseases such as systemic lupus erythematosus
(SLE; see Fig. 38). In the presence of complement (i.e. in fresh serum) only
small complexes are formed; in fact C3 can actually solubilize larger complexes
(see also Fig. 36) and SLE is commonly associated with C2 and C4 deficiency.
Blocking of T-cell or antibody-mediated killing by
complexes in (respectively) antigen or antibody excess may account for some of
the unresponsiveness to tumours or parasite infections.
C1q the first component of complement, binds to the
Fc portion of complexed antibody, possibly under the influence of a
conformational change in the shape of the Ig molecule, although some workers
hold that occupation of both combining sites (i.e. of IgG) is all that is
needed. Activation of the ‘classic’ complement pathway follows.
Inflammation Breakdown products of C3 and C5, through
interaction with mast cells, polymorphs, etc., are responsible for the vascular
damage that is a feature of ‘immune complex diseases’ (see Fig. 36).
Lysis (e.g. of bacteria) requires the complete
complement sequence. Sometimes the C567 unit moves away from the original site
of anti- body binding, activates C8 and 9, and causes lysis of innocent cells,
(e.g. red cells); this is known as
‘reactive lysis’.
Phagocytosis by macrophages, polymorphs, eosinophils, etc.
is the normal fate of large complexes. In general, the antibody classes and
subclasses that bind to Fc receptors also bind to complement, making them
strongly opsonic, but the Fc and C3 receptors are quite distinct; IgM, for
example, binds to complement much more than to cells. The majority of complexes
in the circulation are picked up by red blood cells (rbc in figure) via
their complement receptors (see Fig. 6). In transit through the liver and
spleen, the complexes are removed by phagocytic cells.
Fc receptors (FcR) There are several types of receptor that bind
the Fc constant part of antibodies. Some are found on phagocytes and facilitate
uptake of IgG opsonized bacteria (see Fig. 9). Others are present on
mast cells, and bind IgE. The interaction of IgE and specific antigen then
triggers mast cell degranulation and allergic reactions (see Fig. 35). A
different Fc receptor on B cells binds antibody – anigen complexes and acts to
switch off further antibody production.
Cytotoxicity When antibody bound to a cell or microorganism
makes contact with Fc receptors, the result may be killing rather than phagocytosis.
Cells able to do this include macrophages, monocytes, neutrophils, eosinophils
and natural killer cells (see Fig. 12).
B-cell memory Complement receptors on the follicular
dendritic cells (see Fig. 19) help them to retain immune complexes and present
the antigen to B cells in a way that, by selecting for mutants with high
binding affinity, encourages the increase or ‘maturation’ of the affinity
of the antibody response as a whole.
