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.
