Interest in complement developed at the end of the nineteenth century from observations on cellular and humoral defense mechanisms against bacteria. It was recognized at that time that there were factors in body fluids of animals and man that were capable of killing and lysing bacteria in the absence of cellular factors. Due to the efforts of two of the founders of immunology, Bordet and Ehrlich, and their colleagues, by 1912 the multicomponent nature of complement action was well recognized, the sequence of reaction of the components in the lysis of erythrocytes was defined, complement...