What reagent is used to quench interstitial alkaline phosphatase before primary incubation in IHC?

• A. Water rinse
• B. Weak acid solution prior to primary incubation
• C. Strong base solution
• D. Alcohol wash

Answer: (B)

Quenching endogenous alkaline phosphatase is done to prevent background color from enzymes already present in the tissue when using alkaline phosphatase–based detection. Alkaline phosphatase has its highest activity in basic conditions, so lowering the pH with a weak acid briefly before applying the primary antibody reduces the enzyme’s activity without significantly harming the tissue or antigenicity. This acid pre-treatment helps ensure that any color development comes from the specific antibody–enzyme reaction rather than from endogenous AP.

Water rinsing would not change the pH and hence would not inactivate AP. A strong base would create more alkaline conditions, potentially increasing background rather than reducing it. An alcohol wash might alter tissue morphology but does not reliably inactivate alkaline phosphatase. In some protocols, other inhibitors like levamisole are used, but they’re not the standard approach in this context. Using a mild acidic pre-incubation effectively suppresses background by transiently inactivating the endogenous AP.