There are several serology techniques that can be used depending on the antibodies being studied. These include: ELISA, Haemagglutination Inhibition Test and Complement Fixation.

ELISA

Enzyme-linked Immunosorbent Assay (ELISA) is a biochemical technique used mainly in immunology to detect the presence of an antibody or antigen in a sample. In simple terms, in ELISA an unknown amount of antigen is affixed to a surface, and then a specific antibody is washed over the surface so that it can bind to the antigen. This antibody is linked to an enzyme, and in the final step a substance is added that the enzyme can convert to some detectable signal.

The diagram below shows two different types of ELISA.

Hemagglutination Inhibition Test (HI)

Hemagglutination inibition assay is a clinical lab test used to detect the presence of a a certain hemagglutinating virus or other hemagglutinin antigen based on whether the red blood cells in the sample lose the ability to clump together when the antibody to the virus or other antigen is added to it. If the virus or antigen is present, the antibody kills it and thereby stops it from being able to stick the red blood cells to each other.

The direct measurement of antibody binding to antigen is used in most quantitative serological assays. However, some important assays are based on the ability of antibody binding to alter the physical state of the antigen it binds to. These secondary interactions can be detected in a variety of ways. For instance, when the antigen is displayed on the surface of a large particle such as a bacterium, antibodies can cause the bacteria to clump or agglutinate. The same principle applies to the reactions used in blood typing, only here the target antigens are on the surface of red blood cells and the clumping reaction caused by antibodies against them is called hemagglutination

Complement Fixation

The complement fixation test is based on the use of complement, a biological substance present in the sera of normal animals. Its great value is its predictable activity in the presence of serologically reacting factors and its nonspecificity, that is, it is not like an antibody with a narrow range of reactions or increased concentration occurring in a host following immunization or infection. Furthermore, it is easily destroyed by heating at temperatures that have no deleterious effect on antibodies

It is the nature of complement not to react with an antigen or an antibody alone but to enter into combination with antigen-antibody complexes. The lack of specificity of complement allows it to react with almost any antigen-antibody complex. Therefore, in this test procedure, unless complement is fixed by the particular antigen and antibody system in question, it will remain free to react as an indicator, resulting in a "negative" test. If on the other hand, complement is fixed by the antigen and antibody system under study, it is not free to react in the indicator system, resulting in a "positive" test. The indicator system used in CF is sheep red blood cells (RBCs). In a positive or reactive test, the complement is bound to an antigen-antibody complex, and is not free to interact with sensitized RBCs so they remain unlysed and settle to the bottom of the well to form a button. On the other hand, in a negative or nonreactive test, the complement remains free since there is no antigen-antibody complex for it to bind to, and it interacts with the sensitized RBCs causing them to lyse.

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