Practice question

Radioimmunoassay operates competitively with fixed amounts of specific antibody and radiolabeled antigen. Unlabeled antigen from sample and labeled antigen contend for same binding pocket. When clinical sample contains abundant antigen, it occupies most antibody sites, displacing radioactive tracer into wash fraction. After separation of bound and free fractions, retained radioactivity declines proportionally to sample concentration. Thus calibration curve shows inverse relationship between counts per minute and analyte level. This displacement principle enables precise interpolation of unknown hormone or drug levels using standard curve plotting decreasing bound radioactivity.