RIA stands for:
Radioimmunoassay developed by Solomon Berson and Rosalyn Yalow 1960 represented first method achieving picogram sensitivity for protein hormone quantification, earning Nobel Prize Physiology 1977. Principle competitive: fixed limiting amount of specific antibody incubated with constant amount of radiolabeled antigen tracer and variable unlabeled sample. Radiolabel typically iodine-125 introduced onto tyrosine residues via chloramine T oxidation producing reactive iodonium ion electrophilically substituting aromatic ring or via lactoperoxidase enzymatic iodination preserving biological activity, specific activity up to 100 microcurie per microgram allowing gamma counting, half-life 60 days. Unlabeled analyte competes for antibody binding sites reducing bound tracer proportionally. Separation of bound versus free fractions achieved by precipitation with secondary anti-immunoglobulin antibody forming insoluble immune complex, or polyethylene glycol 6000 increasing macromolecular crowding causing precipitation, or charcoal adsorption of free antigen. Bound radioactivity measured in gamma counter, Scatchard plot analysis yields affinity constant and concentration via dose-response curve. Sensitivity reaches low picogram per ml for insulin enabling discovery of hormone heterogeneity and antibodies in diabetic sera.
Ref: Berson & Yalow J Clin Invest 1960 39:1157 Radioimmunoassay iodine-125; Lodish Molecular Biology competitive binding tracer.