Quantitative Studies of Complement-Fixation

Abstract

Summary Quantitative complement-fixation data are given for the interaction of crystalline bovine serum albumin (BSA) and rabbit anti-BSA. It is shown that the extent of complement-fixation is not a direct linear function of the amount of antibody; graphs of the number of fifty per cent units of complement (C′H50) fixed against micrograms of antibody nitrogen yield an S-shaped curve. A 50 per cent complement-fixation unit (C′F50) is proposed as a measure of the complement-fixing potency of an antiserum. The complement-fixing activity per microgram of antibody N has been found to vary with the length and intensity of immunization; consequently, the C′ fixation titer of an antiserum should not be considered a direct measure of its antibody content. First- and second-course antisera did not yield maximal complement-fixation at the same antibody/antigen ratio, and their complement-fixation curves differed markedly in the zone of antigen excess.