of an individual interaction between a single binding site on an antibody and its target epitope is termed the affinity of that interaction. Avidity and affinity can be judged by the dissociation constant for the interactions they describe. The lower the dissociation constant, the higher the avidity or affinity, and the stronger the interaction. Autoimmune disease Normally antibodies can detect and differentiate molecules from outside of the body and those produced inside the body as a result of cellular activities. Self molecules as ignored by the immune system. However, in certain conditions, the antibodies recognise self molecules as antigens and triggers unexpected immune responses. This results in different autoimmune diseases depending on the type of antigens and antibodies involved. Such conditions are always harmful and sometimes deadly. The exact nature of antibody-antigen interaction in autoimmune disease is not yet understood. Application Antigen-antibody interaction is used in laboratory techniques for serological test of blood compatibility and various pathogenic infections. The most basic is ABO blood group determination, which is useful for blood transfusion. Sophisticated applications include ELISA, enzyme-linked immunospot (Elispot), immunofluorescence, and immunoelectrophoresis. Precipitation reaction Soluble antigens combine with soluble antibodies in presence of an electrolyte at suitable temperature and pH to form insoluble visible complex. This is called a precipitation reaction. It is used for qualitative and quantitative determination of both antigen and antibody. It involves the reaction of soluble antigen with soluble antibodies to form large interlocking aggravated called lattice. It occurs in two distinct stages. Firstly, the antigen and antibody rapidly form antigen-antibody complexes within few seconds and this is followed by a slower reaction in which the antibody-antigen complexes forms lattices that precipitate from the solution. A special ring test is useful for diagnosis of anthrax and determination of adulteration in food. Agglutination reaction It acts on antigen-antibody reaction in which the antibodies cross-link particulate antigens resulting in the visible clumping of the particle. There are two types, namely active and passive agglutination. They are used in blood tests for diagnosis of enteric fever. References External links Antigen-antibody reaction Discover Biotech Immunology Immune system Hematology