Levamisole is used in IHC to block endogenous alkaline phosphatase activity in combination with chromogen, except for intestinal alkaline phosphatase.

• A. To block endogenous ALP in all tissues.
• B. In combination with chromogen to block endogenous ALP except intestinal ALP.
• C. As a primary antibody diluent.
• D. As a mounting medium.

Answer: (B)

Endogenous alkaline phosphatase can cause unwanted background staining when using an alkaline phosphatase–based detection system. Levamisole is added to the chromogen solution to inhibit most of this endogenous ALP activity, thereby reducing background. A key detail is that intestinal alkaline phosphatase is not inhibited by levamisole, so tissues containing intestinal ALP may still show some background. This is why the statement describes blocking endogenous ALP except for intestinal ALP. It’s not used as a primary antibody diluent or as a mounting medium, and it doesn’t block ALP in all tissues.