radioimmunoassay (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 Serum containing the antibodies is withdrawn and treated with a radioactive antigen and later with a nonradioactive antigen.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 Yalow. Yalow was awarded the 1977 Nobel Prize in Physiology or Medicine for her work.
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