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 t-cell 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.