Practice question

Colorimetric immunoassays fundamentally rely on enzyme-substrate chemistry to generate visible product. In ELISA, horseradish peroxidase or alkaline phosphatase linked to antibody catalyzes conversion of TMB or pNPP into colored compound, intensity proportional to antigen amount; no enzyme means no color. Western blot chromogenic detection also uses enzyme-conjugated antibodies acting on BCIP/NBT system to deposit insoluble colored precipitate on membrane. Immunofluorescence, by contrast, depends on fluorochrome excitation and emission, detected as fluorescence under specific filters, not as absorbable color. Therefore no listed method combines colorimetric readout without enzymatic amplification.