Fc epsilon receptor (FcεR) is the Fc receptor that binding IgE. There are two classes of FcεRs on cells of the immune system. The high-affinity receptor, FcεRI, which is capable of binding monomeric IgE. The low-affinity receptor, FcεRII (CD23), which interacts preferentially with complex IgE.
Fc epsilon receptor I is found on epidermal Langerhans cells, eosinophils, mast cells and basophils. The classical Fc epsilon receptor I is tetrameric. It consists of a α-chain which provides the binding site for IgE, β-chain and nomodimeric γ-chain. The β-chain of Fc epsilon receptor I enhances receptor maturation and leads to an increase of FcεRI surface expression and signal transduction capacity within the cells. The γ-chain subunits are responsible for transducing the initial signal into the cells. Fc epsilon receptor I 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 Fc epsilon receptor II are found in B lymphocytes. Fc epsilon receptor II has multiple functions as a membrane-bound or soluble receptor. Fc epsilon receptor II controls B cell growth and differentiation and blocks IgE-binding of eosinophils, monocytes and basophils.