Radiolabelled Antigen (Ag*):

A highly purified antigen is required for radiolabelling without any loss in immunoreactivit y. It acts as a tracer and prepared by introducing the radioisotope to the antigen. Most commonly used radioisotope is 125I (t1/2 = 59.7 d, Eγ = 35 keV) because of its low penetration and availability in high specific activity and radiochemical pure form. Besides, it can be easily introduced into molecules such as tyrosine residues. Because of low energy gammas, it can be counted using thin and small solid scintillation counter. Another radioisotope of iodine 131I (t1/2 = 8 d) can also be used. Other isotopes commonly used are 14C (t1/2 = 5730 years) and 3H (t1/2 = 12.3 years) both of which are β emitters and must be counted by liquid scintillation methods.