Immunohistochemistry (IHC) is a laboratory technique that uses antibodies to detect specific proteins in tissue samples. It is one of the most widely used diagnostic tools in surgical pathology, allowing pathologists to identify what proteins a tissue is expressing and at what level, directly from a stained glass slide.
The technique works by applying a targeted antibody to a thin slice of tissue mounted on a glass slide. If the protein that antibody is designed to detect is present in the tissue, the antibody binds to it. A detection system then converts that binding event into a visible color signal, most commonly a brown stain called DAB (3,3′-diaminobenzidine), so the pathologist can see exactly where and how strongly the protein is expressed under a microscope.
Many cancers are defined not just by how cells look under a microscope, but by which proteins those cells are expressing. IHC allows pathologists to answer questions like: Is this tumor expressing a receptor that a targeted therapy could act on? Has this tumor lost expression of a protein that normally suppresses growth? Is this protein present at normal levels, elevated levels, or absent entirely?
Those answers directly influence treatment decisions, including which therapies a patient is eligible for, whether a tumor is likely to respond to immunotherapy, and how a cancer is formally classified.
A standard IHC slide has two color components. Hematoxylin (blue/purple) is a background counterstain that colors all cell nuclei, giving the pathologist a reference map of the tissue architecture. DAB (brown) is the specific signal that appears only where the target protein is present.
A pathologist reading an IHC slide is looking at where the brown signal appears, how intense it is, and what percentage of cells show it. A slide with no brown staining indicates the target protein is absent or below detection threshold. A slide with strong, widespread brown staining indicates high protein expression.