Cancer vaccines are designed to teach the immune system to attack and destroy cancer cells. Normally, when foreign cells enter the body (for example, when an infection occurs), the immune system responds to the invasion and clears the body of the foreign cells. Unlike infectious cells, cancer cells are not recognized as foreign by the body. Instead, the immune system thinks the cancer cells are part of the normal body and do not mount an immune response against the cancer. Cancer vaccines allow the immune system to recognize cancer cells as foreign and, therefore, get the immune system to attack the cancer cells.
Most commonly, vaccines are used to prevent infections. By introducing inactivated or killed forms of a virus or bacteria to the immune system before an infection actually occurs, the immune system is "primed" to recognize potential infections. Antibodies that are specific for the vaccine are increased in the body and allow for a very rapid response to potential infections by the viruses or bacteria associated with that vaccine. In this way, actual infections can be quickly recognized by the immune system and eliminated before a significant infection can take hold.
Therapeutic cancer vaccines are not designed to prevent cancer. Instead, they are designed to spur the immune system into recognizing tumor cells as foreign invaders so that they may be destroyed by the host immune system. Tumor cells often express distinct antigens known as tumor-associated antigens (TAAs). One of the greatest problems with developing cancer vaccines has been that most TAAs are also present in normal cells. Because the immune system sees these antigens as self-antigens, no immune response is mounted. If the immune system can be taught to recognize the TAAs as foreign, an immune response can be mounted against the tumor. Several TAAs have been identified that are found in specific types of cancers, but not in normal cells. By targeting these TAAs with cancer vaccines, cancer vaccines can induce the immune system to attack cancer cells while leaving normal, healthy cells largely intact. Currently, cancer vaccines targeting cancers of the breast, prostate, liver, kidney, pancreas, and lung, as well as melanoma and certain types of leukemias and lymphomas are in clinical trials.
Some cancers are known to be associated with viral infections. Infection with human papilloma virus has been shown to be a cause of cervical cancer. Hepatitis B and C viruses are known to cause a certain type of liver cancer. Vaccines that prevent infection from these viruses would help to prevent their associated cancers. 1 While these vaccines may ultimately prevent cancer, these are not cancer vaccines. These vaccines are actually vaccines against viruses, rather than cancer itself. Prevention of cancer is merely a consequence of the prevention of the viral infection.