Peroxidase anti-Peroxidase (PAP) complexes are formed by antibodies raised towards Horse Radish Peroxidase (HRP), binding to HRP molecules. Such complexes are used as tertiary detection / signal amplification reagents in immunochemical staining techniques. It must be emphasised that the primary antibody must be raised in the same species as the antibodies used to form the PAP complex, so that a secondary "linker" antibody can be used to "bridge" the primary antibody to the PAP complex. PAP complexes provide a sensitive means of signal amplification, more so than standard direct conjugation of the label to a primary, secondary or tertiary antibody, but not to the degree of avidin / biotin based methods. PAP complexes owe their increased sensitivity to the fact that binding to HRP is facilitated by the antibody's natural affinity for HRP, in oppose to chemical conjugation.