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(RIA), highly sensitive laboratory technique used to measure minute amounts of substances including antigens, hormones, and drugs present in the body. The substance or antigen (a foreign substance in the body that causes antibody production) to be measured is injected into an animal, causing it to produce antibodies. Serum containing the antibodies is withdrawn and treated with a radioactive antigen and later with a nonradioactive antigen. Measurements of the amount of radioactivity are then used to determine the amount of antigen present. The technique was developed by Solomon Berson and Rosalyn YalowYalow, Rosalyn Sussman,
1921–2011, American medical physicist, b. New York City, Ph.D. Univ. of Illinois, 1945. As a researcher at the Bronx Veterans Administration Hospital (from 1947), Yalow and colleague Solomon A.
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. Yalow was awarded the 1977 Nobel Prize in Physiology or Medicine for her work.


A general method employing the reaction of antigen with specific antibody, permitting measurement of the concentration of virtually any substance of biologic interest, often with unparalleled sensitivity. The basis of the method is summarized in the competing reactions shown in the illustration. The unknown concentration of the antigenic substance in a sample is obtained by comparing its inhibitory effect on the binding of radioactively labeled antigen to a limited amount of specific antibody with the inhibitory effect of known standards.

Competing reactions that form basis of radioimmunoassay; * indicates the labeled antigen, and † “in known standard solutions or unknown samplesenlarge picture
Competing reactions that form basis of radioimmunoassay; * indicates the labeled antigen, and † “in known standard solutions or unknown samples

A typical radioimmunoassay is performed by the simultaneous preparation of a series of standard and unknown mixtures in test tubes, each containing identical concentrations of labeled antigen and specific antibody. After an appropriate reaction time the antibody- bound (B) and free (F) fractions of the labeled antigen are separated by one of a variety of techniques. The B/F ratios in the standards are plotted as a function of the concentration of unlabeled antigen (standard curve), and the unknown concentration of antigen is determined by comparing the observed B/F ratio with the standard curve.

The radioimmunoassay principle has found wide application in the measurement of a large and diverse group of substances in a variety of problems of clinical and biological interest. It is therefore not unexpected that there are differences in the specific methods employed for the assay of a particular substance. The full potential of the method has yet to be exploited. It seems that virtually any substance of biologic interest can be measured, the method being modified according to the characteristics of the particular substance. See Antibody, Antigen, Immunology


A sensitive method for determining the concentration of an antigenic substance in a sample by comparing its inhibitory effect on the binding of a radioactivity-labeled antigen to a limited amount of a specific antibody with the inhibitory effect of known standards.
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