Which of the following statements regarding the complement system are true?

a. Complement activation yields products which are directly cytotoxic as well as products which act indirectly via activated leukocytes

d. The alternative and classical pathways converge proximal to generating the membrane attack complex (C5b-9)

The complement system is composed of two different but linked sequences, the classic and alternative pathways. The pathways involve serum proteins that act to amplify the inflammatory-immune response as well as to directly mediate tissue injury. Complement activation by either pathway has been associated with a cascade of events, some of which are mediated directly at a physiologic level by complement products and some of which occur indirectly via activated leukocytes. The direct physiologic effects mediated by C3a and C5a, and to a lesser extent C4a, include increased vascular permeability and contraction of smooth muscle. These are key elements of anaphylaxis. C1 is not an anaphylatoxin as it is the initial complement component which binds to antigen-antibody complex to initiate classical pathway activation. C5a acts principally to alter the behavioral characteristics of leukocytes. Effects include enhanced adherence, enhanced chemotactic activity, release of proteinases, and production of toxic metabolites of oxygen. C3, on the other hand, plays a key role in  bacterial opsonization, resulting in enhanced phagocytosis of invading microorganisms. The alternative and classical complement pathways converge at the C5 level proximal to generating the membrane attack complex (C5b-9) (Figure 6-3).