(A) Key accomplishments (left) and challenges remaining (right) for transplant preparation (3, 4), stem cell infusion (6, 7), and tissue repair after HCT (20, 21, 26). (B) Key accomplishments and challenges remaining for achieving immune homeostasis (11–18) and protecting against immune pathology after HCT (8–10, 22–29). References highlight major JCI articles contributing to the described discoveries.