An immunostain is a specialized laboratory test that pathologists use to identify specific proteins in a tissue sample. The test is performed using a technique called immunohistochemistry (IHC). In this process, antibodies are designed to attach to a particular protein in the cells of the sample. Once the antibody binds, it becomes visible under the microscope through a chemical reaction that alters the color of the cells. This color change is referred to as the immunostain.
Immunostains provide information that cannot be seen on routine stains, such as hematoxylin and eosin (H&E).
They are important because they help pathologists:
Confirm a diagnosis: For example, distinguishing between different types of cancer.
Classify a tumor: Some tumors look similar under the microscope but can be separated by their immunostain pattern.
Guide treatment decisions: Certain immunostains, such as estrogen receptor (ER), progesterone receptor (PR), and HER2, in breast cancer directly influence which treatments will be most effective.
Identify infections: Immunostains can highlight proteins from bacteria or viruses.
Study inflammation: They can show which immune cells are present in inflamed tissue.
In a pathology report, the results of an immunostain are usually described as positive or negative for a specific protein. This means the protein was either detected (positive) or not detected (negative) in the sample. Pathologists often provide more detailed information about the location, extent, and intensity of the staining. These details help doctors better understand the result and its importance.
Common terms include:
Nuclear reactivity: The protein is found in the nucleus (the control center of the cell). This is often seen with markers that affect how cells grow and divide.
Cytoplasmic reactivity: The protein is found in the cytoplasm, the fluid part of the cell surrounding the nucleus. Many enzymes and structural proteins appear here.
Membranous reactivity: The protein is seen along the cell membrane (the cell’s outer edge). This pattern is important for markers like HER2 in breast cancer.
Pathologists also describe how many cells are showing reactivity and how intense the staining is:
Focal reactivity: Only a small group of cells or a limited area shows staining.
Diffuse reactivity: Most or all cells throughout the tissue exhibit staining.
Strong reactivity: The staining is intense and visible, indicating high levels of the protein.
Weak reactivity: The staining is faint, suggesting lower levels of the protein.
By combining these details, a report might say, for example, “strong, diffuse membranous reactivity” or “focal, weak nuclear reactivity.” This type of description helps guide diagnosis and treatment decisions.
ER, PR, and HER2 in breast cancer, which guide therapy decisions.
TTF-1 in lung cancer, which helps identify the tumor’s origin.
p16 in cervical cancer, which is often used as a marker of HPV-related disease.
CD3 and CD20 in lymphoma, which help distinguish T-cell from B-cell lymphomas.
S100, SOX10, and Melan-A in melanoma, which identify tumors that develop from melanocytes.
Which immunostains were performed on my biopsy?
What does it mean that the immunostain was “positive” or “negative”?
How do these results affect my diagnosis?
Are these immunostains important for choosing treatment options?
Will additional immunostains or molecular tests be needed?