Immunoreactivity

Immunoreactivity is a term pathologists use to describe how cells or tissues respond to a special laboratory test called immunohistochemistry (IHC). In this test, antibodies are applied to a tissue sample to look for specific proteins. If the protein being tested is present, the antibody will attach to it, and the cells will change color under the microscope. When this happens, the cells are said to show immunoreactivity.

Why is immunoreactivity important?

Immunoreactivity enables pathologists to identify the type of cells present in a sample and determine whether they are normal or abnormal. Many diseases, including cancers, can be more accurately identified or classified based on the proteins their cells produce. For example, certain lung cancers may show immunoreactivity for the protein TTF-1, while some breast cancers may show immunoreactivity for estrogen receptor (ER) or HER2.

The pattern of immunoreactivity can help confirm a diagnosis, guide treatment decisions, and sometimes provide information about prognosis.

How is immunoreactivity reported?

In a pathology report, immunoreactivity is 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.

Questions to ask your doctor

• Which proteins were tested for immunoreactivity in my biopsy?

• What does it mean that my sample was “positive” or “negative” for this marker?

• How does this result affect my diagnosis?

• Will these findings help decide which treatment is best for me?

• Do I need any additional tests based on these results?