Panel A shows the development of different stable balances between the quantity of persisting donor leukocytes that migrate to host lymphoid organs and the number of antidonor T cells produced at these sites in the organ recipients described in this issue of the Journal.1–3 The greatest opportunity for clonal exhaustion–deletion of the anti-donor response is during the first few days or weeks of maximal leukocyte migration. The gray dashed curve depicts the graft-versus-host (GVH) reaction mounted by immune-competent donor cells that also must be exhausted and deleted. Stabilizing factors may include immunoregulatory cells, antibodies, endogenous molecules, or ongoing maintenance immunosuppressive therapy. Panel B shows the immune responses that occur simultaneously after transplantation. To the extent that reciprocal clonal exhaustion–deletion is not accomplished, one cell population will destroy the other. Solid lines depict the host-versus-graft (HVG) reactions, and dashed lines the GVH reactions. Failure to achieve engraftment with the aid of immunosuppressive therapy implies the inability to control one or both of these responses. Adverse immune events occurred in the patients, but they were reversible.