Abstract: Objective To study the killing effect of polymorphonuclear neutrophils（PMNs） on Trichomonas vaginalis. Methods The vaginal secretion from a patient with vaginitis was incubated in the liver infusion liquid medium to get T. vaginalis. One ml serum was collected from the patient and heated for 30 min at 56 ℃ to inactivate complement in serum, and was absorbed three times with the parasites at 0 ℃ to make the serum free of antibodies. PMNs were separated from the patient’s blood and purified with density gradient centrifugation and polymer accelerating sedimentation. NBT and safranin O were used to stain the sample. The interaction between PMNs and the parasites was observed under microscope. 300 trichomonads and 3×10⁴ PMNs were incubated for 10，20，30，40，50，60 minutes under the conditions of aerobic or anaerobic, with superoxide dismutase （SOD） and catalase （CAT） or without SOD and CAT, and with complement or without complement. They were then inoculated in solid medium for another five days under the anaerobic condition, and surviving organisms were enumerated. Results PMNs were observed to surround and kill a single trichomonad. In the petri-dish containing PMNs, the surviving rate of the parasites in anaerobic condition was 85%， only 3% in aerobic condition （P<0.01）. SOD and CAT reduced the killing effect of PMNs， with a surviving rate of 98% and 94% respectively after 60 min incubation. Without SOD and CAT, the surviving rate is only 2% （P<0.05）. PMNs in the serum without antibodies killed all the parasites，while the complement-inactivated serum fail to kill them. Conclusion The trichomonacidal activity of PMNs relies on the presence of oxygen and complement in the serum of patient.

Key words: Trichomonas vaginalis, Polymorphonuclear neutrophil, Complement, Antibody