antigens

Definition of antigen
National Cancer Institute

antigen

(AN-tih-jen)
Any substance that causes the body to make an immune response against that substance. Antigens include toxins, chemicals, bacteria, viruses, or other substances that come from outside the body. Body tissues and cells, including cancer cells, also have antigens on them that can cause an immune response. These antigens can also be used as markers in laboratory tests to identify those tissues or cells.

Source: NCI Dictionary of Cancer Terms