How does complementary fixation work?

The complement fixation test includes two main parts. The first part involves an indicator system made up of sheep red blood cells, specific antibodies (like immunoglobulin G) that fix complement and an external source of complement, typically obtained from guinea pig serum. In the second part, the patient serum is mixed with a known antigen, and then a complement is added to the mixture. If the patient's serum contains antibodies against the antigen, these antibody-antigen complexes will bind all the complement. Next, sheep red blood cells and anti-sheep antibodies are introduced. If the complement was not used up by the antibody-antigen complexes, it will bind to the indicator system of sheep cells and anti-sheep antibodies. If no antibodies are present in the patient's serum, and thus no antibody-antigen complexes formed, the sheep red blood cells will be lysed resulting in a negative complement fixation test. In contrast if the patient's serum contains complement-fixing antibodies, there will be no red blood cell lysis, indicating a positive test result.