In order to verify that bypass-activated complement can enable polymorphonuclear neutrophils (PMN) to lose their bactericidal capacity,3 experiments were performed as follows:(DA monolayer culture of human PMN was added with zymosan-activated human serum (ZAHS).then the superoxide ion (O2-,the specific granules,and the intracellular bactericidal activity of the cultured PMNs were signigicantly decreased and reached the lowest point in the 6th hour after the adding of ZAHS.0.05 ml of ZAHS showed the maximal effects on the PMNs.(2)Mice were injected intravenously with 0.5 ml of ZAHS and were killed 6 hours later.All the 3 parameters mentioned above dropped markedly in the PMNs isolated from the blood and the lungs of the mice.Pathologically,the lungs of the rats showed acute interstitial inflammation,focal edema,hemorrhage and atelectasis.and subcellular damages on the pulmonary blood-barrier.(3)ZAHS,after neutralization in vitro with antiserum against human C3 and C5,lost most of its harmful effects mentioned above.The findings suggest that the harmful effects of ZAHS originate from the fragments of C3 and Cs and they are most effective when its dosage is suitable and there must be a sufficient length of time to react before the phagocytosis of PMNs.The possible mechanism of the origination of the harmful effects of ZAHS is as follows:Before PMNs undertake phagocytosis,they releas the bactericidal and inflamnagenic (O2-and specific granules extra-cellularly.So the intracellular bactericidal activity of PMNs is decreased.The accumulation of PMNs in the pulmonary tissues can damage the PMNs themselves and the adjacent pulmonary blood-barrier,which will leads to the occurrence of infection and multiple organ failure.