Complement Fixation

Complement fixation is the binding of complement to a complex that is formed during the reaction of an antibody with a specific antigen. Since complement is formed from blood serum only during this reaction, determining the presence of complement after mixing the test serum with the patient’s blood serum can confirm the presence of an infectious disease in a person caused by a suspected microorganism (complement fixation reaction). An example of such a reaction is the Wasserman reaction, which makes it possible to diagnose syphilis.

Complement fixation is the binding of complement to a complex that is formed during the reaction of an antibody with a specific antigen. Since complement is a serum protein that is activated only when this reaction occurs, determining the presence of complement after mixing the test serum with the patient's serum can confirm that a person has an infectious disease caused by a suspected microorganism (complement fixation reaction).

A classic example of such a reaction is the Wasserman reaction, which makes it possible to diagnose syphilis. In this case, the patient's serum containing antibodies to Treponema pallidum, which causes syphilis, is mixed with the test serum and complement is added. If Treponema pallidum antigens are present in the test serum, an “antibody-antigen” immune complex is formed, which binds complement. Complement depletion is judged by the absence of hemolysis of red blood cells added at the last stage of the reaction. Thus, a positive result of the Wasserman test indicates the presence of syphilis.

Article "Complement fixation"

Complement fixation is one of the most common methods for detecting infectious diseases. The essence of the reaction is the binding of complement, an enzyme that plays an important role in the body's immune system. However, if the body suffers from any infectious disease or contact with pathogenic microorganisms, the level of complement in the blood may be reduced.

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