Tumor immunologists have known for decades that the immune system can be an important ally in the fight against cancer. Most early attempts to recruit its potential proved disappointing, however. It turns out that investigators had not done enough to stimulate a key component of the immune system, a kind of master sergeant called the T cell. Without enhancing the ability of T cells both to identify and to attack cancer cells, researchers were, in effect, asking the immune system to go into battle with the biological equivalent of paper airplanes and pellet guns.