BINDING of antigen-antibody complexes to the surface of phagocytes depends on the integrity of the Fc region of the antibody, and occurs even when antibody alone apparently binds only weakly1-4. One possible mechanism is that antigen interacts with antibody to produce an allosteric change in the conformation of the antibody molecule (analogous to that observed by Feinstein and Rowe5), exposing a site capable of interacting with the phagocyte surface. The alternative explanation is that, although "free" antibody molecules have an exposed binding site available for attachment to phagocytes before reaction with antigen, binding is unstable; more stable - and therefore detectable - binding results from the formation of complexes with multiple sites. This possibility does not invoke a change in antibody conformation.
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