What is the indicator used in the complement fixation test?

The complement fixation test is a classical assay used to measure the presence of specific antibodies in a sample by utilizing the complement system's ability to lyse cells. In this test, sheep red blood cells (RBCs) are used as the indicator. This choice is based on the fact that sheep RBCs are readily available and their behavior is well understood in the context of complement activation. When the complement is fixed by the binding of specific antibodies to their antigens, it prevents the lysis of the sheep RBCs. Therefore, if the sheep RBCs are lysed, it indicates that no complement fixation has occurred, suggesting the absence of the specific antibody.

Using sheep RBCs in this context provides a clear and reliable indicator for the test’s results, making it the traditional choice in laboratory settings for complement fixation assays. The other types of RBCs mentioned, such as those from goats, humans, or pigs, do not have the same historical and established utility in this particular test format.