Radioimmunoassay (RIA) is a sensitive laboratory technique used to measure the concentration of antigens in a sample. It involves the use of radioactively labeled antibodies that bind specifically to the target antigen. By comparing the radioactivity of the bound and free antigens, researchers can determine the amount of the antigen present in the sample. This method is widely used in various fields, including clinical diagnostics and biomedical research, to detect hormones, drugs, and other substances in blood or tissue samples. RIA is valued for its precision and ability to measure low concentrations of analytes.