The complement fixation assay can be used to look for the presence of i) specific antibody or ii) specific antigen in a patient's serum. The test utilizes sheep red blood cells (SRBC), anti-SRBC antibody and complement, along with specific antigen (if looking for antibody in serum) or specific antibody (if looking for antigen in serum). If antibody (or antigen) is present in the patient's serum, then the complement is completely utilized and SRBC lysis is minimal. However, if the antibody (or antigen) is not present in the patient's serum, then the complement binds anti-SRBC antibody and lysis of the SRBCs ensues. The following graphics illustrate the complement fixation assay.
