Receptors that recognize the Fc portion of IgG are divided into three groups designated Fc gamma RI, RII, and RIII, also known respectively as CD64, CD32, and CD16. In humans each group of receptors is encoded by 2-3 closely related genes designated A, B, and C. Fc gamma RI binds IgG with high affinity and functions during early immune responses. Fc gamma RII and RIII are low affinity receptors that recognize IgG as aggregates surrounding multivalent antigens during late immune responses. Different Fc receptors can function as activators, as inhibitors, or as decoy receptors.