If antibodies are present, complexes will form and complement will be activated. If no antibodies are present, complement will not be activated. Nothing is visible yet though. SensSRBC's are sheep RBC's which have been reacted with a limited amount of rabbit anti-SRBC's. If complement was activated by the earlier antigen-antibody reaction then the sensitized SRBC's will remain intact and Complement fixation is positive.
Complement fixation is negative.
14.27 shows an example of a CF test for antibodies to Coxiella. Proof that this organism was the one causing the illness. Assays which measure the ability of antibody to neutralize or inhibit microbial activity Mims et. Group A streptococci produce this toxin which has the ability to destroy polys, rbc's and tissue cells adjacent to the site of infection. Antibodies to streptolysin O neutralize this toxin and these antibodies are made in response to Streptococcus pyogenes infections.