The peroxidase-labeled, peroxidase, anti-peroxidase (PAP) antibody method was the pioneer of IHC methods in paraffin-embedded tissues. This method employs an enzyme labeled antibody instead of a fluorescent dye. In the past, fluorescence (FITC) labeled antibodies were the only detection system available to perform antibody-antigen staining and those needed to be performed on frozen sections.
Today, IHC and immunofluorescence (IF) staining can be performed on paraffin-embedded tissue. In addition to the PAP method, there is a phospatase method along the same lines called alkaline phosphatase-antialkaline phosphatase complex (APAAP). These methods were considered to be the most sensitive procedures.