The Leukocyte Cell Surface Receptor(s) for the iC3b Product of Complement

Abstract
The third component of complement (C3) may be activated on a cell surface by either the classical or the alternative complement pathways and is covalently bound to the cell surface as C3b. C3b is a cofactor in the activation of the terminal compoments of C5-C9. In addition, it serves as the ligand for the type 1 complement receptor (CR1). C3b loses both of these functions when it undergoes proteolytic cleavage to iC3b and C3f by factor I. The relatively stable iC3b opsonin may then undergo proteolysis to C3c and C3dg in the presence of serum. Both iC3b and C3dg remain surface bound and are recognized by the leukocyte type 3 complement receptor (CR3) and the type 2 complement receptor (CR2), respectively. Of the surface-bound fragments of C3, iC3b is probably the most stable of the cell-associated C3 fragments and is thought to play a major role in the complement-mediated clearance of micro-organisms.

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