Binding of C1q to IgG or IgM antibody–opsonized pathogens or altered host cells initiates the classical pathway, whereas binding of MBL to sugar motifs on the surface of bacteria initiates the lectin pathway. The alternative pathway is initiated via spontaneous cleavage of the complement protein C3 into C3a and C3b. All three pathways converge onto the complement protein C3, which is cleaved into C3a and C3b by the C3 convertases (C4b2a and C3bCBb). The generation of C3 convertases amplifies the complement cascade through continued cleavage of C3. C3b opsonizes microbes and targets them for destruction by phagocytes. C3b combines with other complement proteins to generate the C5 convertases (C4b2a3b and C3bBb3b), which cleave C5 into C5a and C5b (14, 129). C3a and C5a are complement anaphylatoxins that bind to their respective cell surface receptors, C3aR and C5aR, and promote migration to and activation of immune cells at sites of infection or damage (130, 131). C5b binds to complement proteins C6, C7, C8, and C9 to generate the MAC, which forms a pore in the cell membrane and induces lysis of bacterial or complement-targeted host cells (11). CD55 is a membrane-bound complement regulator that disassembles C3 and C5 convertases to prevent amplification of the cascade and the generation of the MAC (12). CD46, also known as membrane cofactor protein (MCP), works with serum factor I to prevent C3 convertase reassembly and amplification of the cascade. CD46 functions are well characterized in humans but a functional mouse homolog has not yet been identified (132). CD59 is a complement regulator that blocks MAC formation. The complement regulator CR1, which exists in a soluble (sCR1) and membrane-bound form, acts as a receptor for C3b and C4b, thereby destabilizing and enhancing decay of the classical and alternative pathway C3 and C5 convertases (133).