|Detection of Fc epsilon R1 alpha in Human Blood Granulocytes by Flow Cytometry. Human peripheral blood granulocytes were stained with Sheep Anti-Human Fc epsilon RI alpha Antigen Affinity-purified Polyclonal Antibody (Catalog # AF6678, filled histogram) or control antibody (Catalog # 5-001-A, open histogram), followed by Allophycocyanin-conjugated Anti-Sheep IgG Secondary Antibody (Catalog # F0127).|
The alpha subunit of the high affinity IgE receptor (Fc epsilon RI alpha or Fc epsilon RIA) is an IgE‑binding type I transmembrane glycoprotein of the multichain immune recognition (MIRR) family (1, 2). The receptor, Fc epsilon RI, is a tetrameric complex of one alpha, one beta and two gamma subunits ( alpha beta gamma 2) on mast cells and basophils (1). An alternate trimeric form ( alpha gamma 2) is expressed on human, but not rodent, mast cells, basophils, eosinophils and professional antigen presenting cells (3). While the gamma subunit is essential for expression of Fc epsilon RI alpha on the cell surface and for cell signaling, the beta subunit, when present, increases the halflife of the Fc epsilon RI complex on the cell surface (3, 4). An isoform of the beta subunit, beta T, blocks processing of the alpha subunit and its cell surface expression (2, 3, 5). Human Fc epsilon RI alpha cDNA encodes 257 amino acids (aa) including a 25 aa signal sequence, a 180 aa extracellular domain containing two Ig‑like domains that bind IgE and an endoplasmic reticulum retention motif, a 21 aa transmembrane domain with a charged amino acid (Asp219) that contributes to intracellular transport, and a 32 aa cytoplasmic sequence (1, 3, 6). Human Fc epsilon RI alpha shares 50‑62% aa sequence identity with mouse, rat, equine, ovine, bovine, porcine and canine Fc epsilon RI alpha. Binding of IgE alone increases surface expression of Fc epsilon RI, while crosslinking of IgE/Fc epsilon RI complexes by IgE ligands (allergens) initiates receptor internalization and signaling (2, 4, 5). Mast cell and basophil activation by IgE/Fc epsilon RI crosslinking causes degranulation, releasing histamine, leukotrienes, prostaglandins, and other mediators of immediate‑type and late‑phase allergic reactions. Circulating autoantibodies that crosslink Fc epsilon RI alpha are often found in patients with chronic urticaria (7). Fc epsilon RI on human antigen presenting cells mediates uptake and processing of allergens for presentation by class II MHC (2, 3). Fc epsilon RI expression on human DC and Langerhans cells is up‑regulated during allergic reactions (atopy) and correlates with serum IgE concentration (3).