Cytokeratins are proteins made by specialized cells called epithelial cells. These cells are found on the surface of a tissue. This can be an outside surface such as the skin or an inner surface such as the inside of the colon. Epithelial cells also connect together to form channels called ducts and round structures called glands.
Pathologists can perform a test called immunohistochemistry to see the cytokeratin inside of a cell. This test helps a pathologist decide if the cells they are looking at under the microscope are epithelial cells. Cells that produce cytokeratins will be described as positive or reactive. Cells that do not produce cytokeratins will be described as negative or non-reactive.
There are many different kinds of cytokeratins, and the type made by a cell depends on the location of the cell and its purpose. Each type of cytokeratin is given a number, for example cytokeratin 7 or cytokeratin 20. Pathologists often use the short form ‘CK’ followed by a number when describing the results of immunohistochemistry performed on a tissue sample.
Pathologists will often perform immunohistochemistry on a sample of tumour to see if the tumour cells make cytokeratin. Most cancers that make cytokeratins start from epithelial cells. These cancers are called carcinomas and the two most common types of carcinoma are adenocarcinoma and squamous cell carcinoma. Other types of cancer, such as sarcomas and lymphomas, rarely make cytokeratins.
Immunohistochemistry can also help a pathologist decide where the tumour started because the tumour cells usually make the same types of cytokeratins as the normal, healthy cells that started the tumour. For example, the normal healthy cells in the colon make cytokeratin 7 and most tumours that start from cells in the colon also make cytokeratin 7. This is especially helpful when the tumour cells have spread beyond the original tumour to a lymph node or other part of the body. Tumour cells that spread in this way are called a metastasis.