Answer: b, c, d The alternative pathway differs from the classic pathway in that the first steps involving C1, C4 and C2 are bypassed. (See Figure 6-3 previously reproduced.) This pathway can be directly activated by agents other than antigen–antibody complex (e.g., complex polysaccharides like endotoxin and zymosan). Other serum protein factors (e.g., factors B and D) are involved in the activation sequence. Ammonia can attack the thiol-ester, producing amidated C3 and activate the alternative pathway. This leads to membrane attack complex formation (C5b-9) and activation of a number of phagocytic cell functions including toxic oxidant production. This phenomenon may have relevance to several in vivo disease states. In animal models of renal failure, elevated levels of renal vein NH3 have been correlated with impaired renal function and the presence of complement components at the sites of renal injury