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. CD32 proteins bind IgG with lower affinities and recognize IgG aggregates surrounding multivalent antigens. They are expressed on cells of both myeloid and lymphoid lineages as well as on cells of non-hematopoietic origin. Fc gamma RIIA associates with FcR gamma and delivers an activating signal through its cytoplasmic ITAM motif. In contrast, Fc gamma RIIB delivers an inhibitory signal through its ITIM motif. Fc gamma RIIC represents an unequal cross-over event between the IIA and IIB genes.