Efficient inhibition of endogenous peroxidase without antigen denaturation in immunohistochemistry.

The immunoperoxidase technique is commonly used for the antigenic characterization of cells and tissues. However, the occurrence of endogenous peroxidase-positive cells frequently complicates such evaluations. In some instances it is impossible to distinguish between these cells and antibody-stained cells. Methods so far described for inhibiting the endogenous peroxidase, are not always satisfactory - either the endogenous peroxidase is not satisfactorily blocked, or the cellular antigens lose their reactivity. A new two-step method is described which involves transient inhibition of endogenous peroxidase by sodium azide and hydrogen peroxide and a final blocking of this enzyme by lowering the pH to 2.3, while antigens remain undamaged. This method may be of value in immunohistochemical studies of various tissues, especially inflammatory tissues which contain many endogenous peroxidase-positive cells.