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This illustration, and PHIL 14989, depicts what takes place during a complement-fixation test, used to determine the presence of an antigen-antibody complex within a serum sample, and whether or not, with the addition of a complement to the sample, the complement remains free, or in turn, is bound to the antigen-antibody complex. In this case, there is the formation of the antigen-antibody complex, to which the added complement becomes bound, thereby, making it unavailable to bind to, and cause a hemagglutination reaction with sensitized red blood cells (RBCs) when they too, are added to the mixture. The sensitized RBCs have an affinity for the complement, so if there is any free complement in the mixture, it will react with the RBCs causing a hemolytic reaction, as you can see happening in PHIL 14989. This test can be used to detect the presence or absence of either the antigen, or specific antibody in the serum sample.
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CDC/ Dr. Francis Forrester
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