Structure and modulation of Fc and complement receptors.

Recent experiments have revealed the structure of some phagocytosis-promoting receptors. The C3b receptor is a single-chain membrane glycoprotein of Mr 205,000, while the C3bi receptor is composed of two surface glycoprotein chains, of Mr 180,000 and 100,000. Fc receptors all appear to be single-chain glycoproteins of approximately Mr 50,000. Despite this structural similarity, Fc receptors display a broad range of heterogeneity with respect to ligand specificity. One type of Fc receptor (Fc gamma 2b/gamma 1R) appears to function as a ligand-dependent ion channel; the ion flux initiated by the ligation of this receptor may represent the proximal signal sent by this Fc receptor. The second signal sent by other Fc receptors and by the C3 receptors is uncharacterized, except for the observation that the second signal generated by C3 receptors is distinct from that of Fc gamma 2b/gamma 1R.