The immune system does not recognize cancer cells as antigens, or foreign invaders, as it recognizes viruses or bacteria, and so does not fight cancer as it does other diseases. Despite the immune system's inability to recognize cancer cells as foreign, researchers have discovered that cancer cells do carry particular antigens on their surfaces. These antigens may be unique to individual tumors, shared by several tumor types, or expressed by the normal tissue from which a tumor arises.

Vaccine therapy is an experimental treatment that stimulates the immune system to recognize the antigens on cancer cell surfaces. Cancer vaccines are proteins containing cancer cells, parts of cells, or pure antigens, which are injected into the bloodstream. The vaccine stimulates the immune system to:

• Recognize the cancer cells already present in the body as invaders
• Produce antibodies to one or several specific antigens
• Activate cytotoxic T cells to attack the cancer cells that have those antigens

It takes time for the body to build up its own defenses, so the beneficial effects of a vaccine may take months to occur. When successful, however, vaccines promote longer lasting tumor shrinkage than chemotherapy and cause fewer side effects than chemotherapy and other immunotherapies such as interferon and interleukin.

To understand how vaccine therapy works, it helps to understand how the immune system works. See Guide to the Immune System for more information.