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Production of the complement cleavage product, C3a, by activated macrophages and its tumorolytic effects.

Mouse peritoneal macrophages were activated in serum-free culture with lipopolysaccharide, dextran sulphate or C3b. They were found to liberate into the culture medium a cytolytic factor and a factor which was pharmacologically indistinguishable from C3a. The cytolytic activity was neutralized by antiserum against C3a. These results suggest that macrophages can be activated by C3b (or agents that generate C3b) to cleave C3, which they themselves produce. In this way the activated macrophages form C3a, which appears to be a major factor in macrophage-mediated cytolysis. These findings have implications in tumour immunity and in the pathogenesis of inflammatory reactions.