Principle:

It is based on a specific reaction between an antigen (Ag) and an antibody (Ab) by using radiolabelled antigen (Ag*). An antigen is a simple bio molecule with molecular weight of a few hundred Daltons e.g. small steroids, drugs, thyroid hormones, or macromolecules such as virus, bacteria etc. It should be able to react with an antibody, a complex protein bio molecule with molecular weight of over a million Dalton and synthesized within a living system so as to nullify the effect of a foreign attacking substance. For example, if a living system encounters a new virus capable of causing a disease, it will try to resist the virus. One mechanism of resistance is by activation of the immune system to synthesize antibodies. The antibody binds with the antigen, in this case the virus, to form an antigen-antibody (Ag-Ab) complex.

Antigen          →                                                        Antibody production

(Foreign body)                                                                      ↓

                    ↓          Antigen

↓

Antigen-Antibody complex

Inactive                                                                      Ag       Ag-Ab (complex)

+          Ab    ↔

Radiolabelled           Ag*                                         Ag*-Ab

Once antigen is bound to the antibody, the virus is no more active and it is metabolized and excreted from the body. The reaction of the antigen with the antibody is highly specific i.e. an antibody produced against a particular antigen will normally react with that antigen only. This may be compared with the lock and key system made for each other. Moreover, the equilibrium constants of such reactions are very high, of the order of 10⁹ -10¹² dm³ per mole. These antigen-antibody reactions have been used to develop a series of highly specific and sensitive assays known as immunoassay. Some of these assays are capable of measuring analytes down to even femtomole (10^-15M) concentrations in the presence of related analogues.