Quant i ta t ion With Monoclonal Antibodies to H u

Phagocytic cells (neutrophils, monocytes, and macrophages) possess at least two distinct receptors for the third component of complement, C3. Complement receptor type 1 (CR1) 1 is specific for C3b, while complement receptor type 3 (CR3) is specific for the C3b cleavage product iC3b (1). In addition, monocytes may also express a CRy-like receptor that is specific for the d region of iC3b, C3d,g, and C3d (2, 3). CRI and CR3 on neutrophils and monocytes promote phagocytosis of C3-coated particles mainly by promoting adherence of the particle to the phagocyte's surface (4, 5). In contrast, complement receptors (CR) on human and mouse macrophages may promote both binding and phagocytosis of C3-coated particles, depending on the in vitro or in vivo conditions from which the macrophages were obtained (6-10). The data on CR function has been derived from studies using sheep erythrocytes (E) coated with C3 using purified complement (C) components, mouse serum, or human serum. However, little is known about the actual C3 fragments that might be encountered by phagocytes in vivo. Detailed information is available on the sequence of events that occurs after C3b has been fixed to a particle's surface through activation of the classical pathway (CP) or alternative pathway (AP) (reviewed in 11). The several components, C4-binding protein (12), factors H and I (13, 14), and erythrocyte (CR1) (15, 16), play key roles in controlling the activation and propagation of both complement pathways, by their ability to convert bound, hemolytically active C3b into nonhemolytic fragments. In addition, it has become increasingly clear that the surface to which C3b becomes fixed plays a critical role in the susceptibility of C3b to these control proteins. Current data suggests that particles which