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A sensitive reaction used in serology for the detection of either antigen or antibody, as in the diagnosis of many bacterial, viral, and other diseases, including syphilis. It involves two stages: Stage 1 is the binding or fixation of complement if certain antigen-antibody reactions occur, and stage II is detection of residual unbound complement, if any, by its hemolytic action on the sensitized erythrocytes subsequently added (see illustration).
In the first stage either the antigen or the antibody must be supplied as a reagent, with the other of the pair as the test unknown. Fresh guinea pig serum is normally used as a complement source. Sheep erythrocytes which are coated with their corresponding antibody (amboceptor or hemolysin) are used in the second stage. See Complement
The controls A, B, and C in the diagram demonstrate that sufficient complement is present to effect hemolysis of the sensitized indicator cells and that neither antigen or antibody added alone will interfere with this by binding complement. In the test system the combination of a suitable antigen and antibody in the presence of complement will bind the complement to the complex so that the complement becomes unavailable for the hemolysis of the indicator cells added in stage II. If either antigen or antibody is added as a reagent in stage I, then the presence of the other in the test unknown added can be detected through its ability to complete the antigen-antibody system. A lack of hemolysis denotes that the complement is bound. See Immunology, Lytic reaction