IMMUNOLOGY - Roitt A. - Mir 2000

Chapter 9. Antigen Recognition

STRUCTURE OF ANTIGENS

The number of Antibodies with different specificities that can be produced in response to the administration of any antigen is extremely large, since all Antigens are three-dimensional structures exposing A wide variety of configurations to B Cells. The same antigen is often capable of binding antibodies of varying specificities via overlapping epitopes on its surface. Similarly, different antibodies may bind to the same region of an antigen molecule, yet not to the exact same epitope.

Immunodominant epitopes are located on the protruding, flexible Regions of the antigen molecule

The body is capable of producing antibodies against almost any part of an antigen molecule, but during a normal Immune Response this typically does not happen. As a rule, certain epitopes of an antigen exhibit exceptional antigenicity, and the majority of antibodies are directed precisely against them. These epitopes, termed immunodominant, are almost invariably located on the protruding regions of the antigen molecule—specifically, as polypeptide loops that lack a rigid tertiary Structure.

In some cases, immunodominant epitopes are localized within the most flexible regions of the antigen molecule. This has led to the suggestion that the initial configurations of the interacting antigen and antibody molecules are not perfectly complementary to one another. A certain degree of plasticity inherent in both the antigen epitope and the hypervariable regions of the antibody molecule likely facilitates the achievement of an optimal binding energy level. This has been reliably confirmed, in particular, by X-ray crystallographic analysis of complexes formed by antibody Fab fragments with Influenza virus neuraminidase used as an antigen. The parameters and Functional Significance of mutual complementarity for antibody-antigen binding are discussed above (Fig. 9.2, 9.3, and 9.4).