The Division Of Labor
The requirements imposed on the antibody molecule by the two functions are, in a sense, quite opposite. The first function requires great antibody diversity. The second function requires that many different antibody molecules share common features; for instance, it is not practical for Nature to devise a different molecular solution for the problem of elimination of antigens for each different antibody molecule. The conflicting requirements are elegantly met by the antibody structure shown diagrammatically in Figure 3.1.
The structure consists of three units. Two are identical to one another and are involved in binding to antigen. These are the Fab (fragment antigen binding) arms of the molecule. These units contain regions of sequence that vary greatly from one antibody to another and confer on a given antibody its unique binding specificity. The presence of two identical Fab arms enhances the binding of antibody to antigen in the typical situation where multiple copies of antigenic determinants are presented on foreign material.
The third unit – Fc (fragment crystallizable) – is involved in binding to effector molecules. As shown in Figure 3.1, the antibody molecule has a four‐chain structure consisting of two identical heavy chains spanning Fab and Fc and two identical light chains associated only with Fab. The relationship between antigen binding, the different units and the four‐chain structure of the antibody molecule were revealed by a seri s of key experiments that are summarized in Milestone 3.1.