An antigen is a molecule that both can cause the production of a specific type of antibody in the body and that can react with a specific type of antibody. For example, when the immune system recognizes certain foreign (i.e., abnormal) antigens or complexes of antibodies bound to antigens, the immune system may remove the cells containing the abnormal antigen from the body.
Some diagnostic tests for certain types of cancer measure the amount of certain antigens in body fluids or in or on the surface of cells.
Some biological therapies for cancer are monoclonal antibodies, special types of antibodies that bind to specific antigens. For example, Herceptin (generic name, trastuzumab) is a monoclonal antibody that binds to a specific portion of the antigen, HER-2, on the surface of certain types of breast cancer cells.