The therapy, July 5 -- Roughly four in ten people who receive a donor stem cell transplant for blood cancer develop a complication that can be worse than the cancer itself: their new immune system turns on their own body, sometimes for years, sometimes for the rest of their life. On June 30, the Food and Drug Administration approved a therapy designed specifically to change that math.

blood-forming stem cells to rebuild the immune system, a purified dose of regulatory T cells meant to suppress the immune overreaction before it starts, and a calibrated dose of conventional T cells to preserve the graft's ability to kill remaining leukemia cells. Standard transplants deliver all of this as one unsorted mixture and hope the balance works ou...