Which reagent is used to enhance DAB and block pseudoperoxidase activity in IHC procedures?

• A. Hydrogen peroxide
• B. Imidazole 1.01M
• C. Methanol
• D. Sodium borohydride

Answer: (B)

The main idea here is boosting the specific signal from the enzyme-based detection while reducing background from endogenous peroxidases. Imidazole, in a concentrated solution, acts as a peroxidase enhancer for the DAB reaction. It speeds up and amplifies the HRP-catalyzed oxidation of DAB, giving a stronger and more uniform brown deposit where the target antigen is present. At the same time, imidazole helps suppress activity from endogenous peroxidases that would otherwise contribute to nonspecific, pseudoperoxidase background. So you get brighter, more reliable staining of the wanted target with less background noise.

In contrast, hydrogen peroxide is used as the substrate for HRP and is also used to quench endogenous peroxidases before staining, but it doesn’t provide the enhancement and selective suppression in one step. Methanol is mainly a solvent used with peroxide for blocking endogenous activity, not an enhancer. Sodium borohydride is more about reducing autofluorescence and some background elements, not about enhancing DAB signal or suppressing endogenous peroxidases.