There are many kinds of cancer, but treatments have typically combatted them in one way only: by attempting to destroy the cancerous cells. A more recent approach, immunotherapy, co-opts the body’s immune system into attacking and eradicating the tumor. Jerome Groopman, Dina and Raphael Recanati Professor of Medicine at Beth Israel Deaconess Medical Center, authored this article, and William Kaelin, professor of medicine at Dana-Farber Cancer Institute, was quoted.