The Immune System and Cancer

It was not always clear to scientists that the immune system played a role in preventing and combating cancer. This idea was proposed in 1957, but the scientific evidence at the time only seemed to indicate that the immune system protected against pathogens like viruses and bacteria, but not against abnormal body cells like cancer cells. Researchers and doctors in the late 1900s noticed, however, that people with extremely weak or nonexistent immune systems had a greater risk of developing cancer than the average person. In addition, researchers have since noticed that patients with immune cells present in their tumors have a better prognosis than patients without immune cells in their tumors.

Immunosurveillance is a term used to describe the action of the immune cells, including T cells, as they move through the body and look for any abnormalities. When cells become mutated, they may appear to the immune cells as abnormal. The body then recognizes them as non-self or foreign. By eliminating cells that have become abnormal, the immune system helps to protect against cancer. However, if the cells mutate enough so that they are able to escape the surveillance mechanisms of the immune system, they may continue to reproduce as cancer cells. The process is a complex version of 'hide and seek' with major consequences.

As described in the previous pages, T cells recognize peptide antigens 'presented' on their cell surface. If pre-cancerous cells present abnormal proteins T cells will recognize these cells as abnormal. Conversely, pre-cancerous cells that the immune system does not recognize as abnormal, or is unable to kill, will survive and may proliferate to form a tumor.

There are various mechanisms that tumor cells may use to get around the immune defenses of the body. Many cancers produce chemical messengers that inhibit the actions of immune cells. Other cancers have defects in the way that antigens are presented on their cell surface. Other immune cells, called natural killer (NK) cells, play a special role in this case, however, because they notice when body cells no longer have present specific 'self' proteins on their surface and kill the abnormal cells. Additionally, some tumors grow in locations such as the eyes or brain, which are not regularly patrolled by immune cells.

The main goal of immunotherapy and cancer vaccines is to provide the immune system with the signals that it needs to recognize the cancer cells as abnormal. If successful, these strategies may allow the body to recognize and destroy cancer cells, even those that have been able to form a tumor.

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