The activation mechanism of the alternative pathway of the human complement system by the immune precipitate formed with F(ab′) 2 of rabbit IgG antibody: The generation of C3- and C5-cleaving enzymes of the immune precipitate

Abstract

Immune precipitate (F), formed between egg albumin and the F(ab′) 2 fragment of rabbit anti-egg albumin, activated the alternative pathway of the complement system in human serum. On incubation with serum, F combined with several factors in serum (X) to form a complex, FX, which was capable on inactivating purified C3 and C5. FX having maximal activity was obtained by incubation of F with serum for 10–20 min (T max). T max depended on the amount of F added to serum and a increased amound of F shortened the T max/ When FX was incubated at 37°C, its activities decayed as a first order reaction and were completely lost in 120 min. FX regenerated from decayed FX (FX d ) by the addition of both B and D . FX also lost activity by treatment with anti-B or anti-C3. These findings indicated that F(ab′) 2 activated the alternative pathway of the complement system in human serum by the formation of the properdin system enzymes on F.