The possible participation of the alternate pathway of C3 activation in the activation of human heat labile opsonins by bacteria has been investigated. It was shown that: a) opsonization took place normally in human sera completely lacking C1q; b) complete absorption of immunoglobulins and C1q from agammaglobulinemic sera failed to abolish opsonic capacity; c) heat labile opsonic capacity for Escherichia coli was markedly reduced by heating normal serum at 50°C for 30 min, conditions which inactivate a component of the alternate pathway of complement activation and d) addition to such heated serum of purified C3 proactivator, a constituent of the alternate pathway, resulted in restoration of a significant fraction of the opsonic activity. These results indicate that heat labile opsonization is mediated via an alternate pathway of C3 activation.

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