Structural basis for the specificity of antibody–antigen reactions and structural mechanisms for the diversification of antigen-binding specificities

Abstract

The ability of an organism to cope with foreign substances (antigens) depends in part on its capacity to synthesize antibodies (immunoglobulins) of the proper binding specificity to recognize and combine with these antigens. In view of the great variety of possible antigens, antibodies, or more specifically their combining sites, display considerable variation and possess structural properties such as to enable them to bind the antigenic determinants. A vast amount of immunoglobulin sequence data has become available and the three-dimensional structures of a number of immunoglobulin fragments have been elucidated. With these results we can now begin to understand the structural aspects of antibody–antigen reactions. The crystallographic results and the sequence data have been reviewed elsewhere (Capra & Kehoe, 1975; Davies, Padlan & Segal 1975a, b; Gally, 1973; Kabat, 1976; Nisonoff, Hopper & Spring, 1975; Poljak, 1975a, b). Here, an attempt is made to interpret these structural data in terms of the structural evolution of the antibody combining site and the structural basis for the specificity of the binding of antibody to antigen. First, the pertinent crystallographic and sequence data will be presented. Next, the structural studies which reveal the complementarity between antibody and ligand will be described. Then, structural mechanisms by which different combining site structures could be generated will be reviewed.