Fc gamma receptor (FcγR) is a receptor for the Fc portion of IgG. All of the Fc gamma receptors (FcγR) belong to the immunoglobulin superfamily. Fc gamma receptor is essential participant in many immune system effector functions, such as phagocytosis of opsonized, release of inflammatory mediators and antibody-dependent cellular cytotoxicity.
There are three major classes of Fc gamma receptor receptors depending on the affinity of receptor. Fc gamma receptor can be sorted as high-affinity receptor FcγRI (CD64) and low-affinity receptor FcγRII (CD32) and FcγRIII (CD16). The three classes of Fc gamma receptor contain highly conserved extracellular Ig domains.
Fc gamma receptor I (CD64) can be found on monocytes and neutrophils. The most distinctive property of FcγRI is its relatively high affinity for ligand. Fc gamma receptor I binds non-complexed monomeric immunoglobulins. Fc gamma receptor I is the only IgG Fc receptor for which the binding of monomeric ligand can be measured directly. The expression of FcvRI can be enhanced as much as 20 fold by gamma interferon (IFN-γ).
Fc gamma receptor II (CD32) can be found on every FcγR-bearing cell with the exception of the NK cell. The affinity of Fc gamma receptor II for ligand is so low that binding of monomeric ligand either can not be measured directly. As for function, Fc gamma receptor II allows phagocytosis and endocytosis of immune complexes and B cell activation.
Fc gamma receptor III (CD16) is seen only in macrophages and macrophage cell lines, NK cells, myeloid precursor cells and a neutrophil cell line. Fc gamma receptor IIIexpression on macrophages is modulated by IFN-γ. As for function, Fc gamma receptor III mediates antibody-dependent cellular cytotoxicity (ADCC).