What Is Immunohistochemistry (IHC) and Why Is It on My Report?

You received your pathology report and partway through — sometimes in its own section, sometimes embedded in the diagnosis line — you encountered a list of tests with names you do not recognize, followed by results like "positive," "negative," "3+," or percentages. These are your immunohistochemistry results, and they can be among the most difficult parts of a report to understand without context.

The good news: once you understand what IHC is and why it is done, those results start to make sense.

What Is Immunohistochemistry?

Immunohistochemistry — IHC for short — is a laboratory technique that uses antibodies to detect specific proteins in tissue samples. Here is how it works in plain language:

Every cell in your body produces proteins. The proteins a cell produces are determined by what type of cell it is and what it is doing. Cancer cells often produce proteins in abnormal quantities, or produce proteins not normally found in that tissue type. IHC uses highly specific antibodies — proteins that bind only to a particular target — to detect whether a specific protein is present in your tissue, and in what quantity.

The result is a stained microscopic slide where the presence of a protein shows up as a specific color, allowing the pathologist to see precisely which cells are producing that protein and how strongly.

Why Was IHC Ordered on My Tissue?

Pathologists order IHC for several distinct reasons, and more than one may apply to your case:

To Confirm the Diagnosis

Some tumors look similar under the microscope but are biologically different — they arise from different cell types and respond to different treatments. IHC helps the pathologist distinguish between them with greater certainty. For example, a small round cell tumor might look similar to several different cancer types under a standard H&E stain, but an IHC panel can identify which proteins are present and point definitively to the correct diagnosis.

To Determine the Origin of a Metastatic Tumor

When cancer is found at a site away from its original location — in a lymph node, in the liver, in the lung — the pathologist needs to determine where it came from. A breast cancer that has spread to the liver looks different from a primary liver cancer, even though both are in the liver. IHC markers help identify the tumor's tissue of origin, which directs treatment.

To Guide Treatment Decisions

This is where IHC becomes directly relevant to your care. Many cancer treatments work by targeting specific proteins. If your tumor expresses a particular protein, certain targeted therapies may be effective. If it does not, those same therapies will not work. Your physician uses IHC results to personalize your treatment plan.

The most widely known example is HER2 in breast cancer. HER2 is a protein that promotes cell growth. Breast cancers that overexpress HER2 (HER2-positive) respond to targeted anti-HER2 therapies. Cancers that do not overexpress HER2 (HER2-negative) do not respond to those same drugs. IHC is the primary test used to make that determination.

To Assess Prognosis

Some IHC markers provide information about how a tumor is likely to behave over time. The Ki-67 marker, for example, measures cellular proliferation — how rapidly cells are dividing. A high Ki-67 score indicates a more rapidly growing tumor, which factors into prognosis and sometimes treatment decisions.

How to Read IHC Results

IHC results are reported in several ways depending on the marker being tested:

Positive vs. Negative

The most basic IHC result: is the protein present or absent? Positive means the antibody found its target — the protein is present in the tissue. Negative means the protein was not detected. Whether positive or negative is the favorable result depends entirely on what is being tested and why.

Scored Results (0, 1+, 2+, 3+)

For some markers — HER2 is a common example — the result is reported on a scale. The scale reflects both how many cells are positive and how intensely they stain:

• 0 — negative; no staining or barely perceptible
• 1+ — negative; faint, incomplete staining
• 2+ — equivocal; additional testing (typically FISH or ISH) is usually required to clarify the result
• 3+ — positive; strong, complete staining in more than 10% of tumor cells

Percentage

For markers like Ki-67 and hormone receptors (estrogen receptor, progesterone receptor), results are often expressed as a percentage — the proportion of tumor cells that stained positive. A Ki-67 of 80% means 80% of the cells examined were actively dividing at the time the tissue was processed.

Common IHC Markers and What They Test For

What If My IHC Result Is Equivocal?

An equivocal IHC result — most commonly a 2+ on HER2 testing — means the result is not clearly positive or negative, and additional testing is needed to clarify. The most common follow-up test is FISH (fluorescence in situ hybridization) or ISH (in situ hybridization), which examines the actual HER2 gene rather than the protein it produces. These tests provide a more definitive answer in equivocal cases.

If your report includes an equivocal result, ask your physician whether the reflex testing has been ordered and when results are expected.

Questions to Bring to Your Physician

Once you have reviewed your IHC results, consider bringing these questions to your next appointment:

• What specific markers were tested, and what was each one looking for?
• Are my results positive or negative for each marker, and which result is favorable in my situation?
• Do any of my IHC results affect my treatment options — either opening or closing certain therapies?
• Is there any additional molecular testing you would recommend based on my IHC results?
• Are any of my results equivocal, and if so, is additional testing planned?