Immunology Basics - Immunological Computation: Theory and Applications - page 10

B cell receptors (Ab)

Epitopes

Antigen

Figure 1.6 Antigen recognition by B cells. Immune recognition is based on
the complementarity between the binding region of a receptor and an antigen
epitope.

1.3.1 Immune Recognition: Matching and Binding

Several immunological processes require an element (cell or molecule) of the immune
system to recognize the presence of another element. T cell recognition is based
on the complementarity between the binding region of the cell molecule and the
receptor. For instance, antigens are detected when a molecular bond is established
between the antigen and receptors on the surface of B cells (see Figure 1.6). Because
of the large size and complexity of most antigens, only parts of the antigen, discrete
sites called epitopes , get bound to B cell receptors. Multiple receptors bind to an
antigen with varying a nity, that is, the more complementary the structures of the
epitope and the B cell receptor are, the more likely for a stronger bond to occur.
Accordingly, binding or detection in the immune system is approximate to
stimulate a primary response. h is is probably because it is too di cult to evolve
receptor structures that are exact complementary to epitopes (antigen) never
encountered before. If precise binding were required, the chances of a random lym-
phocyte binding to a random epitope would be small. An important consequence
of approximate binding is that a single lymphocyte can detect a subset of epitopes,
which means that fewer lymphocytes are needed to provide protection against a
variety of possible pathogens. h is feature makes the immune system e cient in
terms of time and memory.
A lymphocyte has approximately 10 5 receptors on its surface; because all of
these receptors have the same structure (i.e., a lymphocyte is monoclonal), a single
lymphocyte can only bind to structurally related epitopes. h ese structurally related
epitopes defi ne the similarity subset that a lymphocyte can detect. h e number of
receptors that bind to pathogens determines the a nity of the lymphocyte toward
a given pathogen. If a bond is very likely to occur, then many receptors may bind
to pathogen epitopes, resulting in a high a nity for that pathogen. However, if a
bond is unlikely to occur, then few receptors might bind to epitopes, and the lym-
phocyte will have low a nity for that pathogen. If the lymphocyte's a nity for the