Paradigms for allogeneic stem cell transplantation (A) presently and (B) in the future. (A) Stem cell transplantations are performed with grafts containing stem and progenitor hemopoietic cells and nonspecific donor T cells. Posttransplant immune suppression required to control GVHD promotes opportunistic infection and disease relapse. (B) Stem cell transplantations are performed with grafts containing large numbers of purified stem cells depleted of nonspecific donor T cells. Posttransplant infusions of infection and malignancy specific T cells rapidly reconstitute the immune system in the absence of immune suppression, reducing the risk of opportunistic infection and disease recurrence. (Professional illustration by Alice Y. Chen.)