Immunohistochemistry (IHC) testing for the Human Epidermal Growth Factor Receptor 2 (HER2) protein is a fundamental procedure in modern oncology. This specialized diagnostic test assesses the quantity of HER2 protein present on the surface of tumor cells. The HER2 status acts as a direct indicator of a cancer’s biological behavior and potential response to specific drugs. IHC results are used primarily in managing breast and gastric cancers, helping physicians tailor a treatment plan. Accurate interpretation of the IHC score is a prerequisite for effective, personalized cancer care.
The Function of the HER2 Protein
The HER2 protein, also known as ErbB2, is a member of the epidermal growth factor receptor family found on the outside of many healthy cells. Its normal function is to act as a receptor that receives external growth signals, prompting the cell to grow, divide, and repair itself. This signaling process is carefully regulated to maintain normal tissue function.
In certain cancers, a genetic change called gene amplification occurs, resulting in too many copies of the HER2 gene. This excess genetic material causes cancer cells to produce a significantly higher number of HER2 protein receptors on their surface, a state called overexpression. The overabundance of these receptors constantly floods the cell with growth signals, leading to uncontrolled cell division and rapid growth.
HER2 testing is standard practice for all invasive breast cancers, where approximately 15% to 20% exhibit this overexpression. Testing is also routinely performed for gastric and gastroesophageal junction cancers, where overexpression occurs in about 10% to 30% of cases.
How Immunohistochemistry Works
Immunohistochemistry is a laboratory technique designed to visualize and quantify protein expression within a tissue sample. The process begins by taking a thin section of tumor tissue, typically obtained from a biopsy or surgery, and fixing it to a glass slide. This preparation preserves the cellular structure for microscopic examination.
The core principle of IHC relies on the specific binding affinity between an antibody and its target antigen, the HER2 protein. Specialized primary antibodies are applied to the tissue section, attaching only to the HER2 receptors on the tumor cell membranes. A secondary detection system is then used to make this binding visible to the pathologist.
This detection system often involves adding a chemical dye, such as diaminobenzidine (DAB), which produces a distinct brown color where the antibody-HER2 complex has formed. The intensity and completeness of this brown staining directly correspond to the amount of HER2 protein present on the cell surface. By examining the stained slide, the pathologist assesses the level of protein overexpression, which is translated into a numerical score.
Decoding the HER2 Scoring System
The pathologist assigns an IHC score from 0 to 3+ based on the visual pattern of membrane staining observed in the tumor cells. This standardized scoring system reflects the intensity and completeness of the brown stain on the cell membrane, as well as the proportion of cells stained.
A score of 0 indicates no staining, or faint staining in 10% or less of the tumor cells. A score of 1+ shows incomplete membrane staining that is faint but observed in more than 10% of the cells. Both 0 and 1+ scores are classified as HER2-negative, meaning the protein level is low, and the patient is typically not a candidate for HER2-targeted therapy.
The highest score, 3+, is defined by intense, uniform staining that completely encircles the cell membrane in more than 10% of the tumor cells. This result is deemed HER2-positive, confirming protein overexpression and establishing eligibility for anti-HER2 treatments.
The score of 2+ represents an equivocal or borderline result, characterized by weak to moderate staining that is complete around the cell membrane in more than 10% of the tumor cells. Since this level of protein expression does not definitively confirm overexpression, the 2+ result is inconclusive on its own. This specific result necessitates a mandatory follow-up test to resolve the true HER2 status.
When Confirmatory Genetic Testing Is Required
The 2+ IHC score presents an ambiguity because it shows moderate protein expression that falls short of the clear positive threshold. To clarify the biological status of these tumors, a second, gene-based test is automatically performed on the same tissue sample. This confirmatory testing measures the number of HER2 gene copies inside the cell’s nucleus, rather than the protein on the cell surface.
The most common methods for this clarification are Fluorescence In Situ Hybridization (FISH) and Chromogenic In Situ Hybridization (CISH). FISH utilizes fluorescent probes that attach specifically to the HER2 gene, allowing the pathologist to count the number of gene signals. This count determines if gene amplification, the root cause of overexpression, is present.
The final result is determined by comparing the number of HER2 gene copies to a control gene, resulting in a ratio. If this ratio exceeds a specific threshold, the result is considered FISH-positive, confirming HER2 status regardless of the ambiguous IHC score. If the ratio is below the threshold, the result is FISH-negative, and the initial 2+ IHC score is downgraded to clinically HER2-negative.
How HER2 Status Guides Treatment
The final HER2 status is the most important factor in selecting therapy. A definitive HER2-positive diagnosis makes the cancer susceptible to a class of drugs known as HER2-targeted therapies. These biological treatments work by specifically binding to the overexpressed HER2 receptors on the tumor cell surface, blocking the excessive growth signals.
For patients confirmed as HER2-positive, physicians recommend treatment regimens that include targeted agents, such as trastuzumab or pertuzumab, often administered alongside chemotherapy. The introduction of these specific drugs has improved the long-term outlook for patients with this cancer subtype. Targeted therapy is typically maintained until the disease progresses or unacceptable side effects occur.
Conversely, patients determined to be HER2-negative (0 or 1+ IHC, or 2+ IHC that is FISH-negative) do not benefit from these targeted drugs. Their treatment plan is based on other tumor characteristics, such as hormone receptor status, and typically involves standard chemotherapy, hormone therapy, or alternative systemic approaches. The accuracy of the HER2 test ensures that only patients who can benefit are exposed to the therapy, embodying precision medicine.