complement fixation, an immunological reaction in which an antigen combines with an antibody and its complement, causing the complement factor to become inactive or fixed. The complement-fixation reaction can be tested in the laboratory by exposing the patient’s serum to antigen, complement, and specially sensitized red blood cells. Complement-fixation tests can be used to detect antibodies for infectious diseases, especially syphilis and viral illnesses. They are rarely used in clinical practice today. See also anticomplement, complement, immune system, immunity, Wassermann blood test.
