In the enzyme-linked immunosorbent assay (ELISA), the visible reaction is due to a reaction between

• A. Enzyme and antibody
• B. Enzyme and substrate
• C. Fluorescent dye and antigen
• D. Latex particles and antibody

Answer: (B)

In ELISA the visible signal comes from the enzyme attached to the detection molecule catalyzing a substrate to form a colored product. This enzyme-substrate reaction creates the measurable color change that indicates binding and allows quantification. The antibody–antigen interaction provides the specificity, but the readout itself depends on the enzyme converting its substrate. Fluorescent dyes appear in fluorescence-based assays rather than the standard colorimetric ELISA, and latex particles with antibody are used in latex agglutination, not typical ELISA readouts. Common enzymes like horseradish peroxidase or alkaline phosphatase produce color changes when their substrates are cleaved, giving the visible signal.