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Merck Manual

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Quick Facts

Defenses Against Cancer

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How does your body defend against cancer?

Your body’s immune system can often find a cancer cell and destroy it before the cancer grows or spreads. Your immune system is the network of cells, tissues, and organs in your body that helps protect you from illness and infection. Your immune system can often recognize that cancer cells are abnormal and attack them.

Why do the body’s defenses sometimes fail?

Cancer spreads more easily in people who have a weak immune system or problems with their immune system. This can happen if you:

  • Have HIV or AIDS

  • Take medicines that weaken your immune system

  • Are over 60—as you get older, your immune system doesn’t work as well

However, even when your immune system is working well, sometimes it can’t find and destroy all cancer cells. Sometimes your cancer is too large for the immune system to destroy it.

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