Activation of Alternate Pathway of Complement by Rabbit F(ab′)2 Fragment

Immune precipitate formed with egg albumin and F(ab′) 2 fragment of rabbit anti-egg albumin (F) activated the alternate pathway in normal human serum. F inactivated C3 and C5 without effecting on the early acting components in serum. On incubation with normal human serum, F combined with several factors (X) to form an intermediate complex, designated FX. FX had an ability to inactivate C3 and C5. The time at which FX showed maximal activity to cleave C3 and C5 ( T max ) was 10 to 20 min at 37°C. T max depended on the amount of F added to serum, and increased amount of F shortened T max . FX seemed to be composed of at least three factors, factor B, properdin, and C3, judging from the facts that anti-factor B, anti-properdin, or anti-C3 inhibited C3- and C5-cleaving activity of FX. When preformed FX was incubated at 37°C, it decayed to an inactive form, FX d , at 120 min.