<P> The principles of specificity and cross-reactivity of the antigen - antibody interaction are useful in clinical laboratory for diagnositic purposes . One basic application is determination of ABO blood group . It is also used as a molecular technique for infection with different pathogens, such as HIV, microbes, and helminth parasites . </P> <P> In an antibody, the Fab (fragment, antigen - binding) region is formed from the amino - terminal end of both the light and heavy chains of the immunoglobulin polypeptide . This region called V (variable) domain is composed of amino acid sequences that define each type of antibody and their binding affinity to an antigen . The combined sequence of variable light chain (V) and variable heavy chain (V) creates three hypervariable regions (HV1, HV2, and HV3). In V these are roughly from residues 28 to 35, from 49 to 59, and from 92 to 103, respectively . HV3 is the most variable part . Thus these regions are the paratope, the binding site of antigen . The rest of the V region between the hypervariable regions are called framework regions . Each V domain has four framework domains, namely FR1, FR2, FR3, and FR4 . </P> <P> Antibodies bind antigens through weak chemical interactions, and bonding is essentially non-covalent . Electrostatic interactions, hydrogen bonds, van der Waals forces, and hydrophobic interactions are all known to be involved depending on the interaction sites . </P> <P> Antigen and antibody interact through a high affinity binding much like lock and key . A dynamic equilibrium exists for the binding . For example, the reaction is a reversible one, and can be expressed as: </P>

Study of interaction of antigen and antibody in blood is