Immunoassay is a technique for identifying substances based on the use of antibodies. An antibody reacts with the concerned antigen in a highly specific manner. Antibody reacts only with that determinant of region of antigen for which it is specific.
When proteins react with an antibody, the antigen-antibody complex forms a precipitate, while in case of particulate antigen the complex agglutinates. In either case, the amount of antigen-antibody complex formed or rate of formation is used to determine the type, quantity of antigen or protein. In this way, a substance is identified.
In this technique, the substance, antigen, or protein of interest is immobilized on the surface of a test tube, pertriplate or microtiter well. Now antibody is added allowed to react with protein or antigen. Un-reacted molecules of antibody are washed away and are replaced by other antibody till an antigen-antibody complex is formed.
As the antibodies are antigen a protein specific, thus the substance or the protein is identified taking the antibody into consideration.