What are the common chromogens used in immunohistochemistry (IHC)?

3,3'-Diaminobenzidine (DAB) is the most widely used chromogen in IHC. DAB forms a brown precipitate upon oxidation by enzymes like horseradish peroxidase (HRP), allowing for visualization of the target antigen. Another commonly used chromagen is aminoethyl carbazole (AEC). It produces a red precipitate upon reaction with HRP, providing a contrasting color to DAB staining. Fast Red yields a red precipitate upon reaction with alkaline phosphatase, commonly used in double-labeling experiments or when red staining is desired. 3,3',5,5'-Tetramethylbenzidine (TMB) is a chromogen commonly used in enzyme-linked immunosorbent assays (ELISA) and other immunoassays. TMB produces a blue color upon reaction with HRP, which can be quantified spectrophotometrically. Substrates such as 5-bromo-4-chloro-3-indolyl phosphate/nitro blue tetrazolium (BCIP/NBT) can be used with alkaline phosphatase-conjugated secondary antibodies, resulting in blue or purple precipitates.