What are Immunohistochemical Stains?

When reading a surgical pathology report, you may have wondered exactly what are immunohistochemical (IHC) stains and what are their purpose? Although the basic principle of IHC was described in 1940, it wasn’t until the early 1990s that the method found general application in surgical pathology. IHC is a method to detect specific antigens in cells based on an antigen-antibody reaction which can be recognized at the light microscopic level. The basic procedure involves application of a primary monoclonal antibody directed against a specific tissue antigen. A secondary antibody is then applied which localizes to the first antibody; conjugated to this secondary antibody are molecules of horseradish peroxidase enzyme. Finally, a chromagen, typically diaminobenzidine (DAB), is then applied. (See above) This chemical reacts with the peroxidase and produces a brown reaction product that can be visualized under the microscope, and this is why IHC stains have been referred to as “brown stains”. Interestingly, a major breakthrough in IHC occurred when it was discovered that heating the tissue prior to staining helps uncover antigen sites that were masked during formalin fixation; this was initially known as “microwave retrieval” because the slides were heated in a microwave!

What are IHC Stains Used to Test?

IHC stains are used for a variety of reasons. First, they help the pathologist recognize something that otherwise might be missed, such as a stomach biopsy that may contain rare Helicobacter pylori organisms or a subtle infiltrating adenocarcinoma. IHC stains can help evaluate various differential diagnoses, such as subtyping of a malignant esophageal tumor as being either an adenocarcinoma or squamous cell carcinoma. They can be used to help determine whether a tumor is a primary or metastatic process, and if it is a metastasis, what is the most likely origin.

We also use IHC stains in our mismatch repair gene analysis looking for the loss of protein expression. IHC stains have many other uses, including evaluation of prognostic markers and therapeutic response biomarkers.

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