Complement-Dependent Serological Tests
The complement system is a group of serum proteins that is present in normal serum. The system consists of 20 or more serum proteins that interact with one another and with cell membrane. It is a biochemical cascade that helps to clear patho-gens from the body. It aids the antibodies in lysing bacteria, promoting phagocytosis, and in immune adherence. The com-plement-dependent serological tests may be of the following types:
1. Complement fixation test
2. Immune adherence test
3. Immobilization test
4. Cytolytic or cytocidal reactions
The principle of the complement fixation test is that when anti-gen and antibodies of the IgM or the IgG classes are mixed, com-plement is “fixed” to the antigen–antibody complex. If this occurs on the surface of RBCs, the complement cascade will be activated and hemolysis will occur. The complement fixation test consists of two antigen–antibody complement systems: (a) an indicator system and (b) a test system.
In complement fixation test, “sensitized” red cells are then added to the mixture. If the red cells are lysed, it indicates that there were no antibodies specific to the antigen in the serum of the patient. The complement therefore was not consumed in the test system and was available to be used by the anti-RBC antibodies, resulting in hemolysis. This reaction is considered negative. The test is considered positive if the red cells are not lysed. Nonlysis of the cells indicates that patient’s serum had antibodies specific to the antigen, which have “fixed” comple-ment. Hence, no complement was available to be activated by the indicator system.
Immune adherence test is a test in which certain pathogens (e.g., Vibrio cholerae, Treponema pallidum, etc.) react with specific antibodies in the presence of complement and adhere to eryth-rocytes or platelets. The adherence of cells to bacteria is known as immune adherence, which facilitates phagocytosis of the bacteria.
Immobilization test is a complement-dependent test in which certain live bacteria, such as T. pallidum, are immobilized when mixed with patient’s serum in the presence of complement. This forms the basis of T. pallidum immobilization test. A posi-tive test shows serum to contain treponemal antibodies.
When a live bacterium, such as V. cholerae, is mixed with its specific antibody in the presence of complement, the bacterium is killed and lysed. This forms the basis of test used to measure anti-cholera antibodies in the serum.