ESR spectra of normal human serum after treatment with complement activating agents

We describe the appearance of a free-radical signal in the ESR spectrum of normal human serum incubated with several complement activating agents. The intensity of this signal is dependent of dose of activating agents, time and temperature. Signals elicited by different complement activators differ in morphology and kinetics. Inhibition by treatment with EDTA and the presence of the signal in activated C 6-deficient rabbit serum suggest that the convertase forming steps of complement activation (C2 to 5) could be the source of free-radical containing molecules.