In DAB staining, DAB is oxidized by hydrogen peroxide in a reaction typically catalyzed by horseradish peroxidase (HRP). The oxidized DAB forms a brown precipitate, at the location of the HRP, which can be visualized using light microscopy.

Why do we use peroxidase in IHC?

Peroxidases, most often horseradish peroxidase (HRP), are commonly used as the enzyme catalyst for chromogenic substrate-based staining of target antigens in IHC. Imaging this type of IHC result can be complicated by the presence of endogenous peroxidase and "pseudoperoxidase" activity in the cells and tissues.

How does antibody staining work?

Chromogenic immunohistochemistry: The cell is exposed to a primary antibody (red) that binds to a specific antigen (purple square). The primary antibody binds a secondary (green) antibody that is chemically coupled to an enzyme (blue). The enzyme changes the color of the substrate to a more pigmented one (brown star).