After a cell becomes cancerous, the immune system Overview of the Immune System The immune system is designed to defend the body against foreign or dangerous invaders. Such invaders include Microorganisms (commonly called germs, such as bacteria, viruses, and fungi) Parasites... read more is often able to recognize it as abnormal and destroy it before it replicates or spreads. The cancerous cells may be completely eliminated, in which case the cancer never appears. Certain cancers are 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. Cancers that are more common with a weakened immune system include melanoma Melanoma Melanoma is a skin cancer that begins in the pigment-producing cells of the skin (melanocytes). Melanomas can begin on normal skin or in existing moles. They may be irregular, flat or raised... read more , kidney cancer Kidney Cancer Kidney cancer may cause blood in the urine, pain in the side, or fever. Cancer is most often detected by accident when an imaging test is done for another reason. Diagnosis is by computed tomography... read more , and lymphoma Overview of Lymphoma Lymphomas are cancers of lymphocytes, which reside in the lymphatic system and in blood-forming organs. Lymphomas are cancers of a specific type of white blood cells known as lymphocytes. These... read more . Doctors are not sure why certain other cancers, such as cancers of the lung, breast, prostate and colon, are not more common in people with a weakened immune system.
An antigen is a foreign substance recognized and targeted for destruction by the body’s immune system Overview of the Immune System The immune system is designed to defend the body against foreign or dangerous invaders. Such invaders include Microorganisms (commonly called germs, such as bacteria, viruses, and fungi) Parasites... read more . 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 Breast Cancer Breast cancer occurs when cells in the breast become abnormal and divide uncontrollably. Breast cancer usually starts in the glands that produce milk (lobules) or the tubes (ducts) that carry... read more , ovarian cancer Ovarian Cancer Ovarian cancer, which typically starts on the surface of the ovaries, is not usually diagnosed until it is advanced. Ovarian cancer may not cause symptoms until it is large or has spread. If... read more , and liver cancer Overview of Liver Tumors Liver tumors may be noncancerous (benign) or cancerous (malignant). Cancerous liver tumors are classified as primary (originating in the liver) or metastatic (spreading from elsewhere in the... read more . Vaccines made from tumor antigens are being used to treat prostate cancer and may be able to prevent or treat other types of 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 table Some Tumor Markers Some Tumor Markers* Cancer is suspected based on a person's symptoms, the results of a physical examination, and sometimes the results of screening tests. Occasionally, x-rays obtained for other reasons, such as... read more ).
Even when the immune system is functioning normally, cancer can escape the immune system’s protective surveillance.
One reason the immune system usually does not attack normal cells is that the surface of normals cells carries proteins that signal to circulating immune cells (T-cells) that the cell bearing them is normal and should not be attacked. These are called checkpoint proteins. Sometimes cancer cells develop the ability to produce one or more of these checkpoint proteins and thus escape from attack. Newer types of cancer drugs called checkpoint inhibitors can block the signal and allow the immune system to attack the cancer.