What color does DAB produce when used with HRP?

• A. Brown
• B. Red
• C. Blue
• D. Green

Answer: (A)

When using HRP in immunohistochemistry, the enzyme converts a substrate chromogen into a colored, insoluble deposit at the site of antibody binding. DAB is the chromogen that, in the presence of hydrogen peroxide, is oxidized and polymerizes to form a brown precipitate exactly where the antigen is located. This brown stain provides strong, stable signal that contrasts nicely with a blue hematoxylin counterstain, making the labeled cells easy to see. Other chromogens yield different colors (for example, red with AEC, blue with certain blue substrates), but DAB with HRP reliably produces brown, which is why that color is expected here.