cancer
Defenses Against Cancer
Tumor antigens
An antigen is a foreign substance recognized and targeted for destruction by the body’s 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 large mass of cancerous cells (a cancerous tumor), the body’s immune system may be overwhelmed. Tumor antigens have been identified in several types of cancer, including melanoma, Breast Cancer , Certain tumor antigens can be detected with blood tests. These antigens are sometimes calledtumor markers. Measurements of some of these tumor markers can be used to evaluate people’s response to treatment (see table). 