Antigen (Ag) antibody (Ab) reactions occur when an antigen combines with a corresponding antibody to produce an immune complex. Therefore, an antigen-antibody reaction is thus a bimolecular association which is similar to an enzyme-substrate interaction but the only difference is that antigen-antibody reaction does not lead to an irreversible chemical interaction.
The basis for antigen-antibody reactions are the non-covalent interactions like hydrogen bonds, ionic bonds, van der Waal interactions, hydrophobic interactions, etc. These interactions are individually weak, therefore, a large number of such interactions work together in an antigen-antibody reaction. The in vitro study of antigen antibody reactions is known as serology.
The principle for all diagnostic immunological tests is serological reactions. The binding of an antibody with an antigen of the type that stimulated the formation of the antibody, results in agglutination, precipitation, complement fixation, greater susceptibility to ingestion and destruction by phagocytes, or neutralization of an exotoxin.
The main use of antigen-antibody reactions is in the determination of blood groups for transfusion, serological ascertainment of exposure to infectious agents, and development of immunoassays for the quantification of various substances.
Schematically an Antigen-Antibody Reaction can be represented as:
Ag + Ab [Ag-Ab] → Aggregation → Precipitation/Agglutination/Neutralization
For diagnostic immunological tests, the serological tests must possess high specificity and sensitivity. Specificity is the ability of an antibody to recognize a single specific antigen. There is a high degree of specificity in antigen-antibody reactions.
Antibodies can distinguish differences in:
i. Primary structure of an antigen,
ii. Isomeric forms of an antigen, and
iii. Secondary and tertiary structure of an antigen.
Therefore specificity implies that:
a. Antibody is specific for a single and specific antigen.
b. Antibody wills not cross-react with other antigens.
c. It will not give false positive results.
Sensitivity means the lowest amount of antigen that can be detected. If in a diagnostic test an antibody is capable of detecting a single antigen molecule, then such a test possesses highest sensitivity. The amount of antigen detected in a test is directly proportional to the amount of antibody used. Enzyme Linked Immuno Sorbent Assays (ELISA) is the most sensitive serological tests.