Last updated – 20th March 2021

Education and knowledge is your greatest patient resource.

Knowing what function the IHC test performs will increase your knowledge, your choices, and improve your decision-making. It will also empower you to ask more targeted questions when dealing with medical professionals.

In this article, we’ll show you how easy it is to understand what an IHC test is and what it can do for you.

IHC = “Immunohistochemical”

Immuno – Histo – Chemical (or Chemistry)
This is a testing procedure that analyzes a patient’s biopsy. A biopsy is typically a small tissue sample of the tumor. The biopsy is obtained from either an exploratory procedure where a small piece of the tumor is removed or from the tumor removed during an operation.

The Testing Method.

In a medical laboratory, the biopsy sample is stained with immunoperoxidase (an immunostain) which contains antibodies to known antigen biomarkers. (immune proteins)

The basic principle of this method is that these antibodies will seek out and attach themselves to known (familiar) antigens, that may be present on the cell’s surface. Each antibody is designed to match a specific antigen type – much like a key to a lock. Note; all cells have antigens on their surface.

What is the IHC test looking for?

Known biomarkers:

• What is a biomarker? Biomarkers are typically known as tumor antigens, ( ie PD-L1 & HER2)
• What are Antigens? – (see explanation below)
• MSi-high (Microsatellite Instability High) – (see explanation below)
• Lynch Syndrome – (not covered in this tutorial)

Each primary cancer has its own unique biomarkers, that need to be confirmed. The IHC test also has a particular focus on biomarkers that match current successful targeted and or immunotherapy treatments.

About Antigens

Antigens can vary in type – some are growth receptors and others are ligands that communicate with our immune system. Picture an antigen as a satellite dish made from protein, some dishes are designed to receive messages into the cell, while others are designed to transmit messages from the cell’s surface. IHC tests look for known antigens, this search is often determined by the type of cancer, ie. different cancer types have different target antigens. Antigens are an expression of the cell’s internal activity – a signature.

If an antigen is unknown, (foreign) or a known threat, the immune system’s antibodies will lock on and send a signal back to the T-Cells to initiate an attack – the Antibodies mark these cells for death.

The staining process contains antibodies to known antigens that respond well to targeted and immunotherapy treatments. The antibody’s job is to seek out and bind with their exact antigen match. When a successful binding takes place, the cells change color to highlight this binding.  (source)

The presence of antigens, PD-L1 and HER2 is a positive indicator for ‘Immune Checkpoint Inhibitor’ treatments. (immunotherapy)

MSi -Microsatellite Instability

The IHC test can determine the retention (presence of) or loss of a specific set of genes known as the Mismatch Repair genes. (MLH1, MSH2, MSH6, and PMS2.)

To understand this better, this set of genes together form our bodies “spell-checker” and their job is to repair the mistakes that constantly occur when our DNA replicates itself. These genes together are known as MMR (Mismatch Repair genes.) MLH1, MSH2, MSH6, and PMS2 are protein expressions.

The Loss of 1 or more of these proteins is suggestive of a defective (deficient) DNA ‘Mismatch Repair function’ within the tumor. The MMR is now described as ‘dMMR.’  This loss causes MSi – Microsatellite Instability. If  2 or more genes fail to express (or lost) then this suggests that the Microsatellite Instability is high, and is now described as dMMR/MSi-high. Normally described in results as ‘MSi-High’ –

• Normal Mismatch Repair function is described as ‘MMR.’
• Defective MMR is then described as ‘dMMR’
• ‘MSi-high’ is the result of 2 or more ‘MMR’ genes being deficient.

Note
MSi-high is a positive indicator for ‘Checkpoint Inhibitor’ Immunotherapy treatments.

Summary

If an IHC test is regarded as both time and cost-efficient and takes 3 -5 days to obtain a result. This should be done as soon as the biopsy is obtained. Be sure to ask for the IHC test to include PD-L1, HER2, and MSi status, as this is not always asked for and reported – tissue samples sometimes can only be used once.

The test reveals both MS-high and high levels of PD-L1 (and or HER2) then this matches current ‘Checkpoint Inhibitor’ immunotherapy and targeted therapy treatments, showing breakthrough results.

• Immuno: means the immunological reaction between laboratory-made Antibodies (the good guys) and Antigens (the suspected bad guys)
• Histo: means the ‘Tissue’ sample
• Chemical (or Chemistry): means chemical application. Chemicals that carry the lab-designed antibodies trained to locate specific known biomarkers (antigens)

Also Note;
If an IHC test reveals no targetable Biomarkers, then ask your medical team to order an NGS Genomic profiling of your biopsy sample – this generally takes 4-6 weeks to obtain a result.

Discusses the relationship of High TMB and Checkpoint Biomarkers in relation to immunotherapies