radioimmunoassay (RIA): a procedure for assaying or quantifying minute amounts of target substances such as hormones, neurotransmitters, or drugs in blood or other tissue fluid. It is based on the action of the target substance to bind onto another substance, an antibody, for which it is known to have a special affinity. A measured amount of the target substance is labeled with a radioactive label. It is then mixed with the unlabeled target substance that circulates in unknown quantity in a sample of blood or other fluid, and a measured amount of the antibody is added. {The labeled and unlabeled substances compete to bind on to the antibody.} The amount of unlabeled target substance present can then be quantified, because it varies according to how much of its radioactive rival becomes antibody bound, and the latter can be measured with a radioactivity counter. This is the classical method of antigen competition.

Dictionary of Sexology Project: Main Index

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