The Peroxidase-Antiperoxidase (PAP) method for immunohistochemistry (IHC) staining is an indirect technique used for detecting antigens in tissue samples. In the PAP method, an unlabeled primary antibody binds to the target antigen in the tissue section. This primary antibody is then connected to a PAP complex by a secondary antibody. The PAP complex comprises an antibody against horseradish peroxidase and horseradish peroxidase antigen, both sourced from the same species as the primary antibody. However, the secondary antibody that facilitates this linkage is sourced from a different species but has specificity against the primary antibody.