Which statement best describes a chromogen?

• A. A fluorescent molecule
• B. A fixative
• C. An enzyme
• D. A benzene derivative containing a color-bearing group

Answer: (D)

Chromogenic detection relies on a substrate that, after being acted on by an enzyme linked to the antibody, produces a colored product that marks the location of the antigen. The chromogen is typically a benzene derivative containing a color-bearing group, which after enzymatic conversion forms a colored, insoluble precipitate at the site of interest. A classic example is diaminobenzidine (DAB), which yields a brown color with horseradish peroxidase. This is different from fluorescent dyes used in immunofluorescence, from fixatives that preserve tissue, or from the enzyme itself, which is just the catalyst—not the colored product.