High affinity IgE receptor (FceRI) plays a key role in IgE-mediated allergic immune response. FceRI is a tetrameric receptor complex, which is composed of one a-subunit (FceRIa), one B-subunit, and two y-subunits. FceRIa directly binds IgE with high affinity, while the B- and y-chains are responsible for mediating intracellular signals. FceRIa is a 50kD transmembrane protein with Ig superfamily structure. It is primarily found on mast cells and basophils. Further studies have indicated that FceRIa is also expressed on many inflammatory cells including cutaneuos Langerhans cells, dendritic cells, monocytes of patients with allergic disorders, platelets, bronchial epithelial cells, eosinophils produced in hypereosinophilic syndrome, and neutrophils from allergy-induced asthma patients.