The page below is a sample from the LabCE course Immunohistochemistry (IHC) Basics in Histology. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Peroxidase, Anti-Peroxidase (PAP) Method

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.