Does the immune system protect against cancer?

Dr. Anthony L. Komaroff, MD , Internal Medicine, answered

 The immune system is only moderately effective in fighting cancer. That's because cancer cells are essentially modified "self" cells. That means the immune system usually doesn't recognize them as harmful.

The immune response is triggered only in certain types of cancer -- when cancer cells become so highly abnormal that they are recognizable as "nonself" cells. Sometimes these abnormalities involve changes in proteins made in the cancer cell. When these proteins appear on the cell surface via the major histocompatibility complex (MHC) molecule, the immune system recognizes them as antigens and responds accordingly. But even then, the immune system must race to keep up with the rapidly dividing cancer cells.

One reason cancer cells are difficult to recognize is that they "hide" by removing from their surface the chemical ID that says something is wrong. Even in that case, natural killer (NK) cells -- lymphocytes that act as backup and mobilize when all else fails -- should move in. The job of NK cells is to destroy cells that are suspiciously showing no message. But in cancer, even this fallback may fail. That's because some cancer cells, such as those of the breast, prostate, kidney, lung, and ovary, secrete a pattern of cytokines -- those proteins instrumental in activating the adaptive immune system response -- that somehow interferes with and suppresses the response.

Researchers are working to find ways to enlist the immune system in the fight against cancer. Some are looking at why and how some cancer cells change so much that the immune system recognizes them as "nonself" and treats them as invaders. For such a response to succeed, there must be sufficient numbers of immune fighters to attack the nonself cancer cells. Researchers suggest that with age, the immune system loses some of its firepower in a process known as immunosenescence. They say this could explain why the risk of cancer increases with age, and it may also explain why the elderly are less able to fight off infection.

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