For more than a century, researchers were puzzled by the uncanny ability of cancer cells to evade the immune system. They knew cancer cells were grotesquely abnormal and should be killed by white blood cells. In the laboratory, in Petri dishes, white blood cells could go on the attack against cancer cells. Why, then, could cancers survive in the body? The answer, when it finally came in recent years, arrived with a bonus: a way to thwart a cancer's strategy. Researchers discovered that cancers wrap themselves in an invisible protective shield. And they learned that they could break into that shield with the right drugs.