Opsonic activity in human serum chelated with ethylene glycoltetra-acetic acid.

The influence of the alternative pathway of complement activation on opsonization of S. aureus, E. coli and Candida albicans for phagocytosis was selectively studied by chelating serum with 10 mM EGTA and 10 mM MgC1(2). Opsonic activity of the chelated serum depended on the alternative pathway of complement activation. The classical pathway was inhibited. There was no phagocytosis of S. aureus in chelated normal serum, suggesting that activation of complement activity by the classical pathway is necessary for opsonization of this micro-organism. In control experiments when opsonization of S. aureus was accomplished by IgG separated from hyperimmune serum, without the requirement of complement activity, there was no inhibition of phagocytosis by EGTA and magnesium chloride. E. coli and C. albicans were opsonized in chelated normal serum, indicating that the alternative pathway of complement activation is involved in opsonization of these micro-organisms.