Section 15. Cancer

Chapter 180. Overview of Cancer

Topics: Introduction | How Cancer Develops and Spreads | Types of Cancer | Risk Factors | Bodily Defenses Against Cancer

Bodily Defenses Against Cancer

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, those receiving immunosuppressive drugs, those with certain autoimmune diseases, and older people, in whom the immune system works less well than in younger people. However, even when a person's 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 Section 16, Chapter 183). Antigens are found on the surface of all cells, but normally a person's immune system does not react to his 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, bone cancer (osteosarcoma), and some cancers of the digestive tract. People with these cancers may have antibodies against the tumor antigens. However, the antibodies are usually not powerful enough to control the cancer. In other cancers, such as choriocarcinoma (a cancerous tumor developing in the uterus from parts of a developing embryo), the immune system is much more likely to be able to destroy cancerous cells early.

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 as screening tests in people who have no symptoms of cancer. Sometimes these markers are used for diagnosis and sometimes to evaluate the person's response to treatment (see Section 15, Chapter 181).