Enzyme labels work by exploiting the catalytic activity of enzymes. When an enzyme-conjugated antibody binds to its target antigen in the tissue, the enzyme reacts with a substrate to produce a visible product. Common enzymes used in histology include horseradish peroxidase (HRP) and alkaline phosphatase (AP). For instance, HRP catalyzes the oxidation of diaminobenzidine (DAB), resulting in a brown precipitate that marks the location of the antigen.
