Receptors for the Fc region of IgG (Fc gamma Rs) are members of the Ig superfamily that function in the activation or inhibition of immune responses such as degranulation, phagocytosis, ADCC (antibody-dependent cellular toxicity), cytokine release, and B cell proliferation (1-3). The Fc gamma Rs have been divided into three classes based on close relationships in their extracellular domains; these groups are designated Fc gamma  RI (also known as CD64), Fc gamma  RII (CD32), and Fc gamma  RIII (CD16). Each group may be encoded by multiple genes and exist in different isoforms depending on species and cell type. The CD64 proteins are high affinity receptors (~10^-8-10^-9 M) capable of binding monomeric IgG, whereas the CD16 and CD32 proteins bind IgG with lower affinities (~10^-6-10^-7 M) only recognizing IgG aggregates surrounding multivalent antigens (1, 4). Fc gamma Rs that deliver an activating signal either have an intrinsic immunoreceptor tyrosine-based activation motif (ITAM) within their cytoplasmic domains or associate with one of the ITAM-bearing adapter subunits, FcR gamma or zeta (3, 5). The only inhibitory member in human and mouse, Fc gamma  RIIB, has an intrinsic cytoplasmic immunoreceptor tyrosine-based inhibitory motif (ITIM). The coordinated functioning of activating and inhibitory receptors is necessary for successful initiation, amplification, and termination of immune responses (5).

Three distinct genes encode the human CD32 group, and the protein products are designated Fc gamma  RIIA, B, and C (1). These receptors are glycoproteins of approximately 40 kDa having two extracellular Ig-like domains. The Fc gamma  RII proteins share 94-99% amino acid identity in their extracellular domains but differ substantially in their transmembrane and cytoplasmic domains. Fc gamma  RIIA associates with FcR gamma, and delivers an activating signal upon ligand binding (3, 5). In contrast, Fc gamma  RIIB delivers an inhibitory signal. Fc gamma  RIIC represents an unequal cross-over event between the IIA and IIB genes. Its extracellular domain shares 99% amino acid identity with Fc gamma  RIIB, but a portion of the cytoplasmic domain is closely related to Fc gamma  RIIA. Fc gamma  RII proteins are expressed on cells of both myeloid and lymphoid lineages as well as on cells of non-hematopoietic origin.