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Summary and Explanation: Immunohistochemical techniques have been in use as adjunctive methods for the recognition of cells in tissue sections for at least 50 years. The first method reported use of fluorescent labels in 19421. In 1974, reports of using enzymatic labels, such as horseradish peroxidase (HRP) in routine H & E paraffin sections appeared2,3. The methods have since become the &prime,standard of care&prime, in surgical pathology when classic methods alone fail to yield a definitive diagnosis4-9. Early techniques were based on the peroxidase-anti-peroxidase (PAP) reaction28, while improvements exploited the strong affinity of avidin for biotin10. This technique uses HRP-labeled streptavidin. The secondary antibody is conjugated with biotin and the streptavidin-HRP complex reacts with the biotin on the secondary antibodies. The resulting biotin-avidin-HRP complex can react with the primary antibody bound to the specific epitope of the target antigen. The HRP enzymes of the streptavidin complex then catalyze the substrate/chromogen reaction to form a colored reaction product (brown to black when using DAB as the chromogen) at the antigen site. A biological stain is then used to visualize the whole tissue section.