Background
Receptors for the Fc region of immunoglobin G (FcγR) are divided into three classes and FcγRIII is a multifunctional, low/intermediate affinity receptor. In humans, FcγRIII is expressed as two distinct forms (FcγRIIIA and FcγRIIIB) that are encoded by two different but highly homologous genes in a cell type-specific manner. FcγRIIIB is a low-affinity, GPI-linked receptor expressed by neutrophils and eosinophils, whereas FcγRIIIA is an intermediate affinity polypeptide-anchored transmembrane glycoprotein expressed by a subset of T lymphocytes, natural killer (NK) cells, monocytes, and macrophages. The FcγRIIIA receptor is involved in phagocytosis, secretion of enzymes, inflammatory mediators, antibody-dependent cellular cytotoxicity (ADCC), mast cell degranulation, and clearance of immune complexes. FcγRIIIA has an immunoreceptor tyrosine-based activation motif (ITAM) in its cytoplasmic domain and delivers an activation signal in the immune responses. Aberrant expression or mutations in this gene is implicated in susceptibility to recurrent viral infections, systemic lupus erythematosus, and alloimmune neonatal neutropenia. In humans, it is a 50 -70 kD type I transmembrane activating receptor. The FcγRIIIA cDNA encodes 254 amino acid including a 16aa signal sequence, 191 amino acid ECD with two C2-type Ig-like domains, five potential N-glycosylation sites, a 22 amino acid transmembrane sequence and a 25 amino acid cytoplasmic domain.
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