Practice question

Competitive ELISA measures small antigens where two antibodies cannot bind simultaneously. Wells are coated with reference antigen, sample antigen competes with labeled antigen for limited antibody binding sites. If sample contains abundant target, it occupies antibodies and prevents binding to plate antigen, so labeled complex is washed away and little color develops after substrate addition. Thus strong color indicates antibodies bound plate antigen because no competitor was present, meaning sample was negative for target analyte. Signal is inversely proportional to antigen concentration.