IgG Fc region receptors are members of the Ig superfamily that play roles in activating or inhibiting immune responses such as degranulation, phagocytosis, ADCC, cytokine release, and B-cell proliferation. There are three genes for human Fc γ RII /CD32 (A, B, and C) and one for mouse Fc γ RII B (CD32B). Three distinct genes encode the human CD32 group. These proteins share 94-99% amino acid identity in their extracellular domains but have divergent cytoplasmic domains and signaling capacities. CD32 is a low affinity receptor for IgG. CD32B is expressed on B cells and myeloid dendritic cells. a single gene encodes mouse CD32. The protein product delivers an inhibitory signal upon ligand binding and is functionally equivalent to human Fc gamma RIIB. Mouse CD32 is the only Fc gamma R expressed on B cells and is also expressed on macrophages, neutrophils and mast cells. Mouse CD16 is closely related to mouse CD32 throughout its extracellular domain (95% amino acid identity), but has a divergent cytoplasmic domain and functions as an activating receptor. Together these proteins constitute an activating/inhibiting receptor pair to regulate immune responses.