Fc receptors and immunoglobulin binding factors.

Receptors for the Fc portion of Ig (Fc receptors, FcR) are found on all cell types of the immune system. Three types of FcR react with IgG: Fc gamma RI is a high-affinity receptor binding IgG monomers whereas Fc gamma RII and Fc gamma RIII are low-affinity receptors binding IgG immune complexes; the three types of Fc gamma R are members of the Ig superfamily. Two FcR react with IgE:Fc epsilon RI is a multichain receptor binding IgE with high affinity; it is composed of an IgE-binding alpha chain, homologous to Fc gamma RIII, and of gamma and beta chains that are necessary for receptor expression and signal transduction. The low-affinity Fc epsilon RII is the only FcR described so far that is not a member of the Ig superfamily but resembles animal lectins; it is composed of a transmembrane chain with an intracytoplasmic NH2 terminus. Fc alpha R has homology with Fc gamma R and is a member of the Ig superfamily. Receptors for IgM and IgD are not characterized yet. Finally, Ig transport is made by FcR-like molecules such as the poly-Ig receptor or an MHC-like receptor found on neonatal intestine. A remarkable property of most FcR is the fact that they are released in cell supernatants and circulate in biological fluids as immunoglobulin binding factors (IBF) generated either by cleavage at the cell membrane or by splicing of FcR transmembrane exon. Immunoglobulin binding factors may interfere with Ig-mediated functions and have direct immunoregulatory activities. Involvement of FcR or IBF has been postulated in several diseases, and monoclonal antibodies to FcR are beginning to be used in therapeutics, particularly to target cytotoxic effector lymphocytes and monocytes to tumor cells.