|Description:||This illustration, and PHIL 14988, 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, unlike in PHIL 14988, there is no formation of the antigen-antibody complex, leaving nothing to which the added complement can bind. Therefore, the complement remains available 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 here, but which is absent in PHIL 14988.
This test can be used to detect the presence or absence of either the antigen, or specific antibody in the serum sample.