IHC stands for immunohistochemistry, a laboratory technique that uses antibodies to detect specific proteins in tissue samples. If you’ve seen “IHC” on a pathology report, it means your doctor ordered additional staining on a biopsy to get more precise information about what’s happening in the tissue, most often to diagnose or classify a cancer. It’s one of the most widely used tools in pathology and plays a direct role in determining diagnosis and guiding treatment decisions.
How IHC Works
The core idea behind IHC is straightforward: different cell types produce different proteins. A cancer cell from the lung carries different protein signatures than a cancer cell from the colon. By applying specially designed antibodies to a tissue sample, pathologists can identify which proteins are present, where they’re located, and how much of them there are. The antibodies latch onto their specific target proteins, and a color-producing chemical makes those attachment points visible under a microscope.
Think of it like a lock-and-key system. Each antibody is designed to bind to one specific protein. When it finds its match, the staining process highlights it in a visible color, usually brown or red, against the tissue background. If the target protein isn’t present, the tissue stays unstained. This gives pathologists a visual map of which cells are producing which proteins.
What Happens in the Lab
IHC is most commonly performed on tissue that’s been preserved in a fixative solution and embedded in a wax block. This is the same type of tissue sample collected from a standard biopsy. The process follows a set sequence: first, the tissue is pretreated to “unmask” proteins that may have been altered during the preservation process. Then a primary antibody is applied, which binds to the target protein. A secondary antibody is layered on top to amplify the signal, and finally a detection chemical is added to produce visible color at the binding sites.
Each of these steps needs to go right for accurate results. If the tissue was preserved too long (over 48 hours) or not long enough (under 30 minutes), certain proteins can be damaged or missed entirely. Background staining from antibodies binding nonspecifically can also muddy the picture. Pathology labs run positive and negative controls alongside each test to catch these problems.
Why Doctors Order IHC
The most common reason for IHC testing is cancer diagnosis and classification. Under a standard microscope with basic staining, a pathologist can often tell that a tissue sample is abnormal. But telling exactly what type of cancer it is, or where it originated, requires more detail. IHC fills that gap.
When a tumor is poorly differentiated, meaning the cells look so abnormal they’ve lost the features of their tissue of origin, pathologists use a panel of IHC markers to narrow down the possibilities. A first-pass panel typically includes markers for epithelial cells (carcinomas), immune cells (lymphomas), and pigment-producing cells (melanomas). This alone can sort an unknown tumor into a broad category.
From there, more specific markers help pinpoint the tumor’s origin. Two protein markers called CK7 and CK20, for example, produce distinct patterns depending on whether a cancer started in the lung, colon, ovary, pancreas, or other organs. Among cancers of unknown primary origin, roughly 50% turn out to be adenocarcinomas, 30% poorly differentiated carcinomas, and 15% squamous cell carcinomas, and IHC is the tool that makes those distinctions possible.
Common IHC Markers and What They Mean
If your pathology report lists specific markers, here’s what the most common ones tell your doctor:
- ER and PR (estrogen and progesterone receptors): Used in breast cancer. Tumors that are positive for these receptors are likely to respond to hormone-blocking therapies. Tumors that are negative have a low probability of responding to hormonal treatment.
- HER2: Also used in breast cancer. When this growth-signaling protein is overexpressed, it appears as a crisp staining pattern on the cell membrane. HER2-positive cancers tend to be more aggressive but can be treated with targeted therapies designed to block that protein.
- Ki-67: A proliferation marker that tells pathologists how quickly the tumor cells are dividing. A higher percentage of Ki-67 positive cells generally indicates a faster-growing tumor.
- p53: A tumor suppressor protein. When it’s mutated, it accumulates in the cell nucleus and stains positive on IHC. Alterations in p53 are associated with higher-grade tumors and, for some cancer types, shorter disease-free survival.
Prognostic vs. Predictive Markers
IHC markers fall into two functional categories. A prognostic marker tells your doctor something about the likely course of your disease regardless of treatment. A high Ki-67 score, for instance, signals a fast-growing tumor that may behave more aggressively. A predictive marker, on the other hand, tells your doctor whether a specific treatment is likely to work. ER positivity in breast cancer is a classic predictive marker: it identifies patients who will benefit from hormone therapy. Some markers serve both roles simultaneously.
IHC Beyond Cancer
While cancer diagnosis is its best-known application, IHC is also used to detect infectious organisms in tissue samples. It can identify viruses like cytomegalovirus (CMV) in tissue from transplant patients or people with immune deficiencies, revealing infected cells by highlighting viral proteins in the nucleus. Bacteria like Helicobacter pylori in stomach biopsies, the organism behind Lyme disease, and several species of tuberculosis-causing bacteria can also be detected through IHC when standard staining is inconclusive.
How Long IHC Results Take
Standard biopsy results using basic staining typically come back within two to three days. When IHC is added, expect a longer wait. Studies show the median turnaround time for specimens requiring special stains is about six days, compared to three days for routine specimens. Breast biopsies are a common example: if a malignancy is found, the lab will run ER, PR, and HER2 markers at minimum, adding several days to the reporting time. Lymph node biopsies and kidney biopsies also routinely require multiple IHC stains, which extends the timeline further.
The delay isn’t a sign that something is wrong. It simply reflects the additional steps involved. Each marker requires its own staining run, and the pathologist needs to interpret all the results together before issuing a final report.
Digital Analysis and Automation
Modern pathology labs increasingly use automated staining platforms that standardize the IHC process, reducing variability between runs. Beyond staining, digital pathology tools now allow IHC slides to be scanned into high-resolution images and analyzed by software. These platforms can perform tasks like automated region detection, cell counting, and quantitative scoring of staining intensity, improving both the speed and consistency of interpretation. Tools powered by machine learning are being integrated into diagnostic workflows to assist pathologists with tissue segmentation and diagnostic support, though the final interpretation still rests with a trained pathologist.