The ability of antibodies to specifically recognize a wide variety of antigens with varying affinities reflects the properties of the V regions.
Specificity
Antibodies can be remarkably specific for antigens, capable of distinguishing small differences in chemical structure. The fine specificity of antibodies applies to the recognition of all classes of molecules. For example, antibodies can distinguish between two linear protein determinants differing by only a single conservative amino acid substitution that has little effect on the secondary structure. This high degree of specificity is necessary so that antibodies generated in response to the antigens of one microbe usually do not react with structurally similar self molecules or with the antigens of other microbes. However, some antibodies produced against one antigen may bind to a different, but structurally related, antigen. This is referred to as a cross-reaction. Antibodies that are produced in response to a microbial antigen sometimes cross-react with self antigens, and this may be the basis of certain immunologic diseases.
Diversity
As we discussed earlier in this chapter, an individual is capable of making a tremendous number of different antibodies, on the order of millions, each with a distinct specificity. The ability of antibodies in any individual to specifically bind a large number of different antigens is a reflection of antibody diversity and the total collection of antibodies with different specificities rep resents the antibody repertoire. The genetic mechanisms that generate such a large antibody repertoire are active only in B lymphocytes (and the same mechanisms for generating TCR diversity are active in T cells). This diversity is generated by random recombination of a limited set of inherited germline DNA sequences to form functional genes that encode the V regions of heavy and light chains as well as by the random addition of nucleotide sequences at junctions during the recombination process. The millions of resulting variations in structure are concentrated in the antigen-binding hypervariable regions of both heavy and light chains and thereby determine specificity for antigens.
Affinity Maturation
The ability of antibodies to neutralize toxins and infectious microbes is dependent on tight binding of the antibodies. As we have discussed, tight binding is achieved by high-affinity and high-avidity interactions. A mechanism for the generation of high-affinity antibodies involves subtle changes in the structure of the V regions of antibodies during humoral immune responses to protein antigens. These changes come about by a process of somatic mutation in B lymphocytes that are repeatedly stimulated by antigens and results in new V domain structures, some of which bind the antigen with greater affinity than the original V domains (Fig. 1A). Those B cells producing higher-affinity antibodies preferentially bind to the antigen and, as a result of selection, become the dominant B cells with each subsequent exposure to the antigen. This process, called affinity maturation, results in an increase in the average-binding affinity of antibodies for an antigen as a humoral immune response evolves. Thus, an antibody produced during a primary immune response to a protein antigen often has a Kd in the range of 10−7 to 10−9 M; in subsequent responses to the same antigen (secondary responses), as may occur in repeated infections with the same species of microbe, or repeated immunizations with the same vaccine, the affinity increases, often resulting in a Kd of 10−11 M or even less.
Fig1. Changes in antibody structure during humoral immune responses. The illustration depicts the changes in the structure of antibodies that may be produced by the progeny of activated B cells (one clone) and the related changes in function. (A) During affinity maturation, mutations in the V region (indicated by yellow dots) lead to changes in fine specificity without changes in C region–dependent effector functions. (B) In class switching, the C regions change (indicated by color change from purple to green, yellow, or blue) without changes in the antigen-binding V region. Class switching is seen in membrane-bound and secreted antibodies. (C) Activated B cells may shift production from largely membrane-bound antibodies containing transmembrane and cytoplasmic regions to secreted antibodies. Secreted antibodies may or may not show V gene mutations (i.e., secretion of antibodies occurs before and after affinity maturation). Ig, Immunoglobulin.
