ABSTRACT
IgGs are the most abundant immunoglobulins, and perhaps for that reason, the term "Fc receptors" typically refers to the three families of IgG binding proteins encoded in the human genome and expressed constitutively by one or more specific cell types. These three families, FcyRI, FcyRII, and FcyRIII, contain eight distinct genes and generate even more protein products by alternative splicing within several of the genes. However, the human genome also encodes for receptors for other immunoglobulin classes (lgA, IgD, IgE, and IgM) as well as for Fc receptors with specialized functions in immunoglobulin transport (the neonatal Fc receptor, FcRn; and the polymeric Ig receptor). Although our level of knowledge about each of these other receptors varies, it is certainly likely that at least several of them play an important role in vascular diseases. Intuitively, the IgA receptor (FcaR, CD89) probably plays an important role in the vasculitis of Schonlein-Henoch purpura, and the IgE receptor (Fc£R, CD23) clearly plays a role in the vascular permeability of some urticarias and possibly urticarial vasculitis. In addition to all of these endogenously encoded receptors, it is also important to remember that cells infected with certain herpes viruses can express virally encoded proteins that have Fc receptor-like binding capacity (Fig. 1). These proteins, whose true functions are unknown, raise interesting possibilities for the role of antibodies and immunologically relevant cells in virally induced vascular injury.
