“I was told that it was career suicide,” said cancer researcher Marcela Maus of her decision to study immunotherapy decades ago. “People had been trying for a hundred years to get the immune system to recognize cancer, and it was a dead end.” Earlier this year, Maus and her team at Mass General in Boston proved the skeptics wrong, injecting T cells designed to recognize tumors into the brain fluid of three glioblastoma patients, whose tumors radically and rapidly regressed. It is the latest good news from the trenches of immunotherapy research, which has already led to promising treatments for leukemia and melanoma. Features editor Christopher Cox went to Boston to understand the treatment’s long journey from theoretical prospect to lab-tested reality — and the promising possibilities that lay just over the horizon.