Fc receptors are found on a number of cells in the immune system including phagocytes like macrophages and monocytes, granulocytes like neutrophils and eosinophils and lymphocytes of the innate immune system (natural killer cells) or adaptive immune system (e.g., B cells). They allow these cells to bind antibodies that are attached to the surface of microbes or microbe infected cells, helping these cells to identify and eliminate microbial pathogens. Activation of phagocytes is the most common function attributed to Fc receptors.
Fc alpha receptor is a member of the multichain immune recognition receptor (MIRR) family. Fc alpha receptor is composed of two extracellular Ig-like domains, α-chain and γ-chain. Fc alpha receptor I γ-chain contains a so-called ITAM (immunoreceptor tyrosine-based activation motif) signaling motif. Fc alpha receptor I with ITAM activates sequentially src family tyrosine kinases and syk family tyrosine kinases and mediates endocytosis, ADCC and recyling of IgA. Signaling is dependent on association of FcαRI with another FcαRI γ-chain subunit, forming the trimmer FcαRIα/γγ.
Fc gamma receptor (FcγR) is a kind of Fc receptor that bind IgG. Depending on the affinity of receptor, Fc gamma receptor is classified as high-affinity receptor FcγRI, low-afffinity receptor FcγRII and FcγRIII. FcγRII allows phagocytosis and endocytosis of immune complexes and B cells activation. FcγRIII is a transmembrane molecule expressed in NK cells and activated monocytes on which it mediates antibody-dependent cellular cytotoxicity (ADCC).
Fc epsilon receptor is the Fc receptor expressed in granulocytes and binding IgE. The same as Fc gamma receptor, Fc epsilon receptor can be sorted as high-affinity receptor FcεRI and low-afffinity receptor FcεRII. Among them, FcεRI is involved in allergic reactions and defense against parasitic infections. When an appropriate allergic antigen or parasite is present, the cross-linking of a least two of IgE molecules and their Fc receptors on the surface of a granulocyte will trigger the cell to rapidly release preformed mediators from its granules. Most FcεRII are found in B lymphocytes. FcεRII mediates endocytosis of IgE-antigen complex by B lymphocytes and phagocytosis of IgE-coated particles.