Immunohistochemistry (IHC) is a special test that pathologists perform on tissue to help them make the correct diagnosis. Immunohistochemistry 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 performed. For this reason, immunohistochemistry 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.
When the tests works well, only the cells which 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).
There are thousands of probes available to identify thousands of different chemicals. This variety allows pathologists to distinguish between cells that otherwise look identical. 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 protein in your tissue, the result is called ‘positive’ or ‘reactive’. When no protein is identified, the result will be described as ‘negative’ or ‘non-reactive’.