Fc Receptor Homologs (FcRH1-5) Extend the Fc Receptor Family

Receptors for the Fc portion of immunoglobulins (FcR) are expressed broadly among cells of the immune system (Ravetch and Kinet 1991; Daeron 1997). The differential expression of these Ig isotope-specific receptors enable them to modulate cellular and humoral immunity by linking their antibody ligands with effector cells. These cellular receptors have the ability to sense humoral concentrations of antibody, initiate cellular responses in host defense, and participate in autoimmune disorders (Ravetch and Bolland 2001). The diverse regulatory roles of FcR depend upon their Ig isotype specificity, cellular distribution, and cytoplasmic signaling elements. The different FcR molecules share similarities in their ligand-binding subunits and overall extracellular structures, but they differ in their transmembrane and cytoplasmic regions in two major ways. These receptors either have self-contained inhibitory or activating signaling motifs or instead they use charged residues in their transmembrane region to pair with signal-transducing adaptor chains possessing activating signaling motifs.