In the war on cancer, researchers have long struggled to recruit the body's immune system to attack tumors while leaving healthy cells alone. This strategy-basically that of vaccination-would offer a huge improvement over existing treatments such as chemotherapy and radiation, which kill healthy and diseased cells indiscriminately and carry harsh side effects. But repeated attempts to find a cancer vaccine have failed, largely because scientists have had a crude understanding of the molecular mechanics of the immune system and cancer cells.

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