B cells are small white blood cells that contribute to our immune defenses. They produce antibodies, which help us to recover from infections. B cells may cause diseases when they misbehave. If they mutate and then divide out of control, B cells can produce blood cancers called leukemias or tissue cancers called lymphomas. This article traces the development of Rituximab, a drug that kills cancerous B cells. Paradoxically, the drug is itself an antibody—the product of a B cell! Developing such an antibody into a drug required deep knowledge of the immune system, clever genetic engineering, and great effort by scientists and medical doctors. This is a story of several great scientists building on each other’s work to create a drug that has saved many lives.