FcERI, also known as FCER1A, is the alpha subunit of the immunoglobulin epsilon receptor (IgE receptor). IgE receptor is a high affinity IgE receptor which plays a central role in allergic disease, coupling allergen and mast cell to initiate the inflammatory and immediate hypersensitivity responses that are characteristic of disorders such as hay fever and asthma. The allergic response occurs when 2 or more IgE receptors are crosslinked via IgE molecules that in turn are bound to an allergen (antigen) molecule. A perturbation occurs that brings about the release of histamine and proteases from the granules in the cytoplasm of the mast cell and leads to the synthesis of prostaglandins and leukotrienes--potent effectors of the hypersensitivity response. IgE receptor is comprised of an alpha subunit(FcERI), a beta subunit, and two gamma subunits. FcERI is glycosylated and contains 2 Ig-like (immunoglobulin-like) domains.