FCERI is primarily expressed on mast cells and basophils as a tetrameric complex comprising an IgE-binding alpha subunit, a signal amplifying membrane-tetraspanning beta subunit, and a disulfide-linked gamma chain dimer that provides the receptor its signaling competence (Blank & Rivera 2004). In the absence of an antigen or allergen, FCERI receptor binds to monomeric IgE antibodies, and thus the receptor adopts the antigenic specificity of the prevalent IgE repertoire (Garman et al. 2000). Mast cell activation is initiated when multivalent antigen crosslinks the IgE bound to the high-affinity FCERI, thereby aggregating FCERI (Siraganian 2003). Antigen driven aggregation of FCERI then elicits intracellular signals that result in mast cell exocytosis.