Episode 004: Heme Path Series Pt. 3 - Immunohistochemistry

Major Points Covered:

• In Part 3 of our Heme Path series, we break down the basics of immunohistochemistry (IHC)

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Take Aways:

1. What is Immunohistochemistry?

   • A molecular technique that utilizes fluorescently-labeled probes or chromogenic reporter stains to identify populations of cells. The goal being to help identify clonality. 

   • Procedure entails taking thin slices of tissue and then using the probes to look for the presence (or absence) of cells specific to that probe

   • Helps to identify specific proteins, the pattern and distribution of which can help identify what it is that we are looking at

2. Pros of IHC: 

   • Can be performed on fixed specimen (vs. flow cytometry, which requires live cells) 

   • Provides an idea of micro-architecture of the sample (diffuse distribution of cells vs. clusters) in context

3. Cons of IHC: 

   • More labor intensive than flow, therefore can take longer to result 

   • Need to have an index of suspicion for what you are looking for. Your search is only as good as the probes you use! 

4. Examples of utility:  

   • Diagnosis of acute leukemia: For instance, staining for CD34, which is expressed on blasts, can help you determine the “blast percentage.” That is, you count up number of cells that stain for CD34 probe then divide by total number of cells in the sample, which gives blast percentage. 

   • Diagnosis of plasma cell dyscrasias: These disorders result in “sticky” bone marrows, such that it is hard to get a good aspirate. This means on flow, you may not get an accurate percentage. Using IHC helps you better estimate the numbers. 

   • Diagnosis of lymphoma: Staining for specific markers (for example CD20, CD19 for B-cell lymphomas) can help in diagnosis. Can also use IHC to determine KI67, which is the proliferation index. 

   • Diagnosis of metastatic cancers: For instance, if you see a lung mass but also note axillary lymphadenopathy, so you sample the lymph node because less invasive. How you do you know if the cells in the lymph node are representative of metastatic lung cancer cells? Use IHC to stain them!

   • PDL-1 status: This is believed to be important marker to predict response to immunotherapy.

References

1. https://www.abcam.com/content/immunohistochemistry-the-complete-guide - Great description of process and images to showcase what IHC staining looks like (not endorsement of product)