CVF–induced AP and anti-OVA/OVA–induced classic pathway complement activation. Western blot analysis of C3 activation in wild-type (A) or properdin^−/− (B) mouse serum. Cleavage product of the C3 α-chain was detected in serum treated with CVF in Mg²⁺-EGTA but not in untreated serum or serum treated with CVF in EDTA. (C) Densitometry of cleaved and intact C3 α-chain in panels A and B. (D) ELISA plate assays of anti-OVA/OVA–induced classic pathway complement activation in wild-type (WT), properdin^−/−, and factor B knockout (fB^−/−) mouse serum or in properdin^−/− serum treated with an antihuman fB antibody.