How antibodies work: focus on Fc receptors.

It is increasingly appreciated that the part of an antibody not involved in the binding of antigen--the Fc region--plays an important biological role. It activates a variety of receptors not only on so-called effector cells such as macrophages and granulocytes, but also on lymphocytes, and it can thereby modulate the immune response itself. Over the past 2 years much new information has been gained about the structure of such receptors, in large part through molecular genetics. In this review we describe the structure and some aspects of the function of the most complicated of the cellular Fc receptors so far identified: the receptor with high affinity for immunoglobulin E (IgE) on mast cells. The structure of its IgE-binding chain is strikingly similar to the corresponding polypeptide of an immunoglobulin G receptor. Like the latter and like a receptor that binds polymeric immunoglobulin, segments of the protein resemble immunoglobulin sequences. It is surprising that other IgE-binding proteins that putatively serve related functions have completely different structures.