Antigens are those substance that stimulates the production of antibodies which, when enter into the body it reacts specifically in a manner that are clearly visible.
Some antigens may not induce antibody production, but instead creates immunological tolerance.
An antigen introduced into the body produces only specific antibodies and will react with only those specific antigens.
These antibodies appear in the serum and tissue fluids. All antibodies are considered as immunoglobulin. They are mainly of five classes; IgG, IgA, IgM, IgD and IgE.
Antigen- antibody reactions are known as serological reactions and are used as serological diagnostic tests for the identification of infectious diseases.
The reaction occurs mainly in three stages;
1. The initial interaction between the antigen and antibody, which produces no visible effects. It is a reversible and rapid reaction.
2. The secondary stage leads to the demonstration proceedings, such as precipitation, agglutination, etc.
3. The tertiary reaction follows the neutralization or destruction of injurious antigens. These results in clinical allergy and other immunological diseases.
There are certain characteristics for antigen-antibody reactions;
1. Reaction is specific.
2. The whole molecules participate in the reaction, and not just a part of it.
3. No denaturation of antigen or antibody occurs during the reaction.
4. The combination usually occurs at the surface.
5. The combination is firm, but reversible
6. Agglutinins formed after agglutination usually are formed by both antigen and antibody together.
7. They can combine in varying proportions.
Measurement of antigen and antibody are made in terms of mass or as units or titre.
Serological reactions include;
1. Precipitation reaction; a soluble antigen combining with the specific antibody in the presence of electrolytes at a suitable temperature and pH forming insoluble precipitins.  Commonly used tests are ring test, slide test, tube test, immunodiffusion, etc.
2. Agglutination reaction; when a particulate antigen is mixed with its antibody in the presence of electrolytes at a suitable temperature and pH, the particles are clumped or agglutinated.
3. Complement fixation test (CFT); the ability of antigen antibody complexes to fix complement is made use in this test. Complement is something which takes part in any immunological reaction and absorbed during the combining of antigen with its specific antibody. The best example of CFT is the Wassermann reaction done for the detection of Syphilis.
4. Neutralization test; different types of these are available. Virus neutralization, toxin neutralization, etc. are some of its kind.
5. Opsonization; this makes use of the determination of opsonic index, which is the ratio of the phagocytic activity of patient’s blood to the phagocytic activity of the normal patient’s for a given bacterium.
6. Immunfluorescence, the method of labeling the antibodies with fluorescent dyes and using them for the detection of antigens in tissues.
7. Radioimmunoassay (RIA); is a competitive binding radioisotopes and enzymes are used as labels to conjugate with antigens or antibodies.
8. Enzyme Immuno Assay (EIA); the assays based on the measurement of enzyme labeled antigen or antibody. The most common example is ELISA used to detect HIV.
9. Immunoelectroblot; it uses the sensitivity of Enzyme immunoassay with a greater specificity. Example is Western blot done for the serodiagnosis of HIV infection.