What IHC Testing Is and Why It Matters
IHC testing, or immunohistochemistry, is a laboratory technique that uses antibodies to detect specific proteins in cells within a tissue sample. It is a cornerstone of modern pathology, helping clinicians identify the type of disease, predict how it will behave, and determine which therapies are most likely to work. Because it works on preserved tissue sections, IHC bridges the gap between a microscope and a molecular diagnosis, giving clinicians actionable information without requiring fresh or frozen samples.
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How IHC Testing Works
The process starts with a thin slice of tissue, usually from a biopsy or surgical specimen, which is fixed, embedded in paraffin, and mounted on a slide. The lab applies a primary antibody that binds to the target protein, then a secondary antibody linked to an enzyme or fluorescent label. When a substrate is added, the label produces a visible color or signal at the site of the protein, which a pathologist reads under a microscope. The result is reported as the percentage of stained cells, the intensity of staining, and the location within the cell, all of which contribute to the interpretation.
Common Clinical Applications
Pathologists use IHC testing to classify tumors when the origin is unclear, distinguish between benign and malignant growths, and identify specific molecular targets that qualify a patient for certain therapies. It is widely used in breast cancer for HER2 status, in lymphoma for lineage markers, and in melanoma for markers like S100 and SOX10. IHC also supports the diagnosis of infectious diseases, autoimmune conditions, and hereditary cancer syndromes when paired with other tests.
Strengths and Limitations
IHC testing is valued for being relatively fast, cost-effective, and compatible with routine pathology workflows. It can be performed on archived tissue, enabling retrospective studies and second opinions years after a biopsy. However, results depend on tissue quality, antibody specificity, and staining protocols, so pre-analytical factors matter. False positives and false negatives can occur, and IHC results are often interpreted alongside molecular tests, imaging, and clinical history to reach a final diagnosis.
The Role of IHC in Personalized Treatment
By identifying protein expression patterns, IHC testing helps match patients with targeted therapies and clinical trials. A positive or negative result can be the deciding factor between a standard chemotherapy regimen and a biologic agent designed for a specific biomarker. As antibody panels and automated staining platforms improve, IHC continues to expand its role in precision medicine, making it one of the most widely used tests in the diagnostic pathology lab.