Three classes of IgE-binding proteins have been described. This review summarizes recent data on what is generally known as the low affinity receptor for IgE (FceRII). The two other types of IgE binding proteins the high affinity IgE receptor (FceRI) found on mast cells and basophils and lymphocyte-produced IgE-binding factors-are discussed only with regard to their potential relationships to the FceRII or the soluble form of the FceRII. For additional information, other reviews on these IgE binding components are recommended (1, 2). The FceRII is found on a variety of cell types; thus, the function may be quite diverse. Several other recent reviews on the FceRII are also recommended (3, 4).

Discovery The first demonstration of a cell type other than the mast cell or basophil having receptors for IgE was by Lawrence et al (5). Radiolabeled myeloma IgE aggregated with F (ab'h fragments of rabbit antihuman Fab'was shown to bind to human peripheral blood lymphocytes. Further rosette analyses and direct binding studies demonstrated that several B lym phoblastoid cell lines as well as human peripheral blood B lymphocytes possess the FceRII (6). As discussed below, at least in humans, the FceRII is found on a variety of hematopoietic cell types.