The complement system has several functions: 1. Opsonisation - C3 convertase converts C3 into C3a and C3b, with C3b binding to pathogens to stimulate phagocytosis. 2. Lysis of pathogens - C3 convertase generates C5 convertase, converting C5 to C5a and C5b, with C5b combining with other factors to form the membrane attack complex that ruptures bacterial cell membranes. 3. Chemotaxis - C5a and C3a attract neutrophils and macrophages to the infection site by causing extravasation from capillaries.