Complement-fixing antibody response in pneumococcal pneumonia

Previous studies of complement-fixing antibodies to pneumococcal capsular polysaccharides in humans have yielded conflicting results. We studied 65 sera from 25 patients with pneumococcal pneumonia, using both fresh sera and heat-inactivated sera with added human complement. Only 4 of the 25 patients developed detectable levels of complement-fixing anticapsular antibody. Of the 25 patients, 22 developed detectable levels of hemagglutinating anticapsular antibody, indicating that they were able to develop an immunological response during the infection. Most of the antibody detected by hemagglutination was sensitive to 2-mercaptoethanol, but some 2-mercaptoethanol-resistant antibody was also detected. In studies with rabbit antiserum, the complement fixation test was found to be as sensitive as the hemagglutination test for detection of anticapsular antibody. It is not clear why detectable levels of complement-fixing antibody do not develop more often in patients with pneumococcal pneumonia. Studies of purified anticapsular antibody would be of interest to determine whether or not these antibodies are restricted to immunoglobulin subclasses having a limited capacity to fix complement.