Complement fixation by antibodies to the alpha toxin of Staphylococcus aureus

When rabbits were injected with the heat-denatured alpha toxin (toxoid) of Staphylococcus aureus, the immune response was demonstrated by an increase in antitoxin that fixed complement. Such antitoxin was detected in 72% of normal human sera. After fractionation of the antitoxin into two types (the antibinding antibodies and the indirect hemagglutinating antibodies), both types of antibodies were found to fix complement in the standard serological complement fixation test. In addition, the indirect hemagglutinating antibodies were capable of fixing complement when the antigen (alpha toxin or toxoid) was covalently or noncovalently bound to erythrocyte membranes. The fixation of complement by membrane-bound immune complexes did not result in lysis of the carrier erythrocytes. The prevalence of complement-fixing antitoxin in normal humans and animals raised the concern that the outcome of in vivo experiments involving alpha toxin could be influenced by the immune status of the host.