Studies on Immunity in Anthrax VIII. Relationship of Complement-Fixing Activity to Protective Activity of Culture Filtrates

Summary Complement fixation reactions were obtained with rabbit antisera and culture filtrates or partially purified preparations of protective antigen of Bacillus anthracis . By selection of appropriate antisera and adjustment of conditions, fixation by heat-stable antigens of no protective activity could be minimized or eliminated. Fixation titers were compared with the activity of the antigen preparations in active immunization of guinea pigs. A method was developed for deriving a dose-response curve from the mean reciprocal survival time after challenge of immunized guinea pigs. By use of this curve the results of the immunization assays were converted to relative potency units of protective antigen. There was significant correlation between the complement fixation titers and the immunizing activity of the preparations. The deviations were no greater than would be expected in view of the limited precision of the in vivo assay when practical numbers of guinea pigs are used. The significance of the results is discussed.