Structural Bases of FcγR Functions

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Abstract
This review describes structures which determine the biological activities triggered by FcγR and account for the cell-mediated functions of IgG antibodies in physiology and pathology. The binding specificity and affinity of FcγR depend primarily on IgG-binding structures, in their immunoglobulin-like extracellular domains. Binding is however also influenced by subunits that associate to multichain FcγR. Effector and regulatory intracytoplasmic sequences that are unique to molecules of the FCγRIIB family determine the internalization properties of these receptors. Immunoreceptor Tyrosine-based Activation Motifs (ITAMs) are intracytoplasmic effector sequences shared by FcγR and other receptors involved in the recognition of antigen, which trigger cell activation and internalization. Immunoreceptor Tyrosine-based Inhibition Motifs (ITIMs) are intracytoplasmic sequences, shared by FCγRIIB and a growing number of negative coreceptiors which negatively regulate cell activation via ITAM-bearing receptors. Altogether, these structures enable IgG antibodies to exert a variety of finely tuned biological effects during the immune response.