Publisher Summary The name complement was given to describe the heat-labile property of normal human serum that “complements” the action of antibody in producing the immune lysis of red blood cells and bacteria. It soon became apparent that complement was highly complex biochemically and functionally. There are thymus-independent antigens, such as polyvinylpyrolidone, that do not activate complement at all. Waldmann and Lachmann were unable to produce inhibition of antibody response in vitro in the Michell–Dutton system by interfering with complement either by the use of cobra venom factor or by the depletion of C3b inactivator. Cobra venom factor is indeed highly immunosuppressive in vitro, but this effect is due to phospholipase A contamination, and highly purified cobra venom fector that has full complement activating activity is without immunosuppressive effects in the Michell-Dutton system. Klaus and Humphrey have gone on to show that the prolonged decomplementation prevents the generation of B cell memory and that it is likely to be a reflection of the failure to localize antigen in germinal centers.