Immunohistochemistry (IHC) is a special test that pathologists perform on a tissue sample to help them make the correct diagnosis. This test allows pathologists to highlight groups of cells based on the chemicals those cells are making. Immunohistochemistry is usually performed on tissue after the more common H&E stain has been performed. For this reason, this test is considered an ancillary test (a test that supports other tests).
Immunohistochemistry works by attaching a probe (an antibody) to the chemical of interest (the antigen). Importantly, the probe will only stick to cells that contain the chemical of interest. A second probe is then added to the tissue which makes the cells containing the initial chemical change colour. There are thousands of probes available to identify thousands of different chemicals. This variety allows pathologists to distinguish between cells that otherwise look identical.
When the tests works well, only the cells that contain the chemical of interest should change colour. When viewed under a microscope the target cells stand out in sharp contrast to the unstained cells in the background. Most immunohistochemical tests cause the target cells to turn brown or red (see image above). Immunohistochemistry also allows pathologists to identify specific proteins that can help predict the behavior of a disease or the response to drugs such as chemotherapy.
When an immunohistochemistry test identifies a chemical in the tissue sample, the result is called positive or reactive. When no chemical is identified, the result will be described as negative or non-reactive. Sometimes the location of the chemical within the cell will also be described. Cytoplasmic staining means that the chemical was seen within the body or cytoplasm of the cell. Nuclear staining means that the chemical was seen in the nucleus of the cell, the part that holds most of the cell’s genetic information. Finally, membranous staining means that the chemical was seen on the thin barrier that surrounds the outside of the cell.