What is a chromagen?

• A. A benzene derivative containing a color-bearing group
• B. A dye that stains all tissues
• C. A buffer solution
• D. A type of enzyme

Answer: (A)

Chromogens are colored compounds used in immunohistochemistry that, when activated by an enzyme in the detection system, produce a visible color at the site of antigen binding. These molecules are typically aromatic, often benzene derivatives, containing a color-bearing group (chromophore) that develops color after the enzyme-driven reaction. This structural idea—a benzene-based molecule with a color-producing group—is why the statement describing a benzene derivative containing a color-bearing group correctly defines a chromogen. In practice, enzymes like horseradish peroxidase oxidize the chromogen substrate to form a colored precipitate (for example, DAB yields brown, AEC yields red). The other options describe a general dye, a buffer, or an enzyme, none of which capture the color-generating role of a chromogen.