Practice question

Freshly collected fetal bovine serum contains intact complement system cascade components of innate immunity capable of opsonization, chemotaxis, and direct lysis. Components include classical pathway C1 complex C1q six headed collagen-like recognizing immune aggregates, C1r C1s serine proteases cleaving C4 C2 forming C3 convertase C4b2b, alternative pathway factor B D, lectin pathway mannose binding lectin MBL associated serine proteases MASP, central protein C3 three chain alpha beta gamma 185 kDa convertible to active C3b depositing via thioester covalent attachment onto cell surface hydroxyl, and terminal pathway C5 convertase cleaving C5 into C5b initiating assembly C6 C7 C8 C9 forming membrane attack complex MAC transmembrane pore ten nanometers causing osmotic lysis, plus anaphylatoxins C3a C5a five to ten kDa inducing degranulation chemotaxis via G protein coupled receptors C3aR C5aR increasing vascular permeability. In culture, complement present in non-heat inactivated serum can lyse xenogeneic cells, promote background cytotoxicity in antibody mediated complement dependent cytotoxicity assay masking specific killing, and trigger unwanted inflammation via inflammasome activation.