The document discusses enzyme-linked immunosorbent assay (ELISA), a technique used to detect substances like proteins. ELISA uses an enzyme to detect the binding of antibodies to antigens, producing a color change that indicates the presence of the antibody-antigen complex. There are direct and indirect ELISA methods, as well as variations like immobilized antigen ELISA for antibody detection and sandwich ELISA for antigen detection. ELISA has applications in medical diagnostics to detect antibodies in patients and test for substances like allergens in food.