Activation of complement component C3 by any of three initiation pathways (classical, alternative, and lectin) leads to cleavage of this molecule with generation of the biologically active fragments C3a and C3b. C3b and its further sequential cleavage fragments, iC3b and C3dg/C3d, are ligands for complement receptors 1 and 2 (CR1 and CR2) and the β2 integrins, CD11b/CD18 (CR3) and CD11c/CD18 (CR4), which are present on a variety of phagocytic and immune accessory cells. 1–6