Antibody-antigen complex equilibrium constant

S. Sensitivity of the Radioimmunoassay. This depends on two factors. First, the hormone must be labeled with suflScient radioactive nuclide to allow the counting of free and antibody bound hormone fractions to be statistically significant at the end of the incubation period. Second, provided that the protein can be labeled to a high enough specific activity, the sensitivity of the test will depend on the equilibrium constant of the antibody-antigen system in use. The higher the constant in favor of the complex formation, the more sensitive will be the assay. 

Antibodies form complexes with their respective antigens (here analytes). These complexes can show a particular strength, which can be quantified by the affinity constant (or equilibrium constant). This affinity constant is about 10 -10 L mol for most analytically useful antibodies. The higher this number, the more stable is the complex. The highest known affinity in the biochemical field is the interaction between avidin (an egg protein) and biotin (vitamin H), for which a value of around 10 L mol has been determined. The affinity constant plays an important role in immunoassays and other immunological techniques. The development of new methods is greatly facilitated if this constant is known. 

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