Even when a cell becomes cancerous, the immune system is thought to be able to recognize it as abnormal and destroy it before it replicates or spreads. Cancer is more likely to progress in people whose immune system is altered or impaired, as in people with AIDS, people receiving immunosuppressive drugs, people with certain autoimmune disorders, and older people, in whom the immune system works less well than in younger people. However, even when the immune system is functioning normally, cancer can escape the immune system's protective surveillance.

Tumor Antigens: An antigen is a foreign substance recognized and targeted for destruction by the body's immune system (see Biology of the Immune System: Overview of the Immune System). Antigens are found on the surface of all cells, but normally the immune system does not react to a person's own cells. When a cell becomes cancerous, new antigens—unfamiliar to the immune system—appear on the cell's surface. The immune system may regard these new antigens, called tumor antigens, as foreign and may be able to contain or destroy the cancerous cells. This is the mechanism by which the body destroys abnormal cells and is often able to destroy cancerous cells before they can become established. However, even a fully functioning immune system cannot always destroy all cancerous cells. And, once cancerous cells reproduce and form a mass of cancerous cells (a cancerous tumor), the body's immune system is highly unlikely to be able to destroy it.

Tumor antigens have been identified in several types of cancer, including malignant melanoma. Vaccines made from tumor antigens might be able to prevent or treat cancer by stimulating the immune system. Such vaccines are an area of great research interest.

Certain tumor antigens can be detected with blood tests. These antigens are sometimes called tumor markers. Measurements of some of these tumor markers can be used to evaluate people's response to treatment (see Selected Tumor Markers*). However, except for the marker prostate specific antigen (PSA), tumor markers are not very helpful as screening tests in people who have no symptoms of cancer.

Last full review/revision August 2008 by Bruce A. Chabner, MD; Elizabeth Chabner Thompson, MD, MPH