FC-RECEPTORS AND THEIR INTERACTIONS WITH IMMUNOGLOBULINS

Receptors for the Fc domain of immunoglobulins play an important role in immune defense. There are two well-defined functional classes of ma mmalian receptors. One class of receptors transports immunoglobulins a cross epithelial tissues to their main sites of action. This class inc ludes the neonatal Fc receptor (FcRn), which transports immunoglobulin G (IgG), and the polymeric immunoglobulin receptor (pIgR), which tran sports immunoglobulin A (IgA) and immunoglobulin M (IgM). Another clas s of receptors present on the surfaces of effector cells triggers vari ous biological responses upon binding antibody-antigen complexes. Of t hese, the IgG receptors (Fc gamma R) and immunoglobulin E (IgE) recept ors (Fc epsilon R) are the best characterized. The biological response s elicited include antibody-dependent, cell-mediated cytotoxicity, pha gocytosis, release of inflammatory mediators, and regulation of lympho cyte proliferation and differentiation. We summarize the current knowl edge of the structures and functions of FcRn, pIgR, and the Fc gamma R and Fc epsilon RI proteins, concentrating on the interactions of the extracellular portions of these receptors with immunoglobulins.