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Fc receptors

Fc receptors
(Science: immunology) receptors for the fc portion of immunoglobulins.
Fc-R (30 kd) is the receptor for serum and secretory IgA1 or IgA2 and is expressed on most myeloid cells and subpopulations of t and B-cells. Fc-RI (CD64) high affinity receptor (72 kd on gels) for monomeric IgG1 found on monocytes, macrophages and some neutrophils. The extracellular portion has three immunoglobulin superfamily C2 domains, in contrast to Fc-RII, Fc-RIII that have only two. Involved in antibody dependent cell killing and in clearance of immune complexes. Fc-RII (CD32) low affinity receptor 40 kd) for aggregated igg that exists in several sub types coded by three closely related genes, a, B and c. All forms are found on monocytes, the B forms (that are alternatively spliced) are found on B-cells, the a and c forms are present on neutrophils. Binding of aggregated igg will trigger phagocytosis and the oxidative burst in neutrophils. Fc-RIII (CD16) is the low affinity receptor (50-80 kd on gels) for aggregated igg. It is found in transmembrane and gpi linked forms. The transmembrane form associates with the – subunit of Fc-RI or the TCR- chain and on B-cells with the – chain of Fc-RI. Has structural similarity with Fc-RI, Fc-RII and Fc-RI-. Binding of aggregated igg or igg antigen complexes mediates phagocytosis or antibody dependent cellular cytotoxicity. Fc-RI is a heteromeric high affinity receptor for ige found on mast cells and basophils. The – chain (45-65 kd on gels, 25 kd of polypetide) is N glycosylated and has two immunoglobulin C2 loops in addition to the transmembrane domain, the – chain (32 kd) has four transmembrane domains, the – subunit is a homodimer (8 kd monomer) identical to the – subunit of CD16 and has similarity with – and – chains associated with the tcell receptor. Binding of antigen to the ige Fc-R complex triggers the release of histamine and various inflammatory mediators. Fc-RIIa & b CD23) low affinity receptor (45 kd) for ige. Both a and b are present on mature B-cells, the b form on monocytes, il-4 activated macrophages, eosinophils, platelets and dendritic cells. The protein has a c type lectin domain that mediates ige binding and can be cleaved from the membrane to yield an active soluble form.