Expressions of allotolerance. Top: Experimental and clinical models in which tolerance is associated with donor leukocyte macrochimerism. Bottom: Organ and composite tissue transplants for which alloengraftment is contingent on persistent donor leukocyte microchimerism. Middle: Tolerance permutations defined as balances between the quantity of persisting donor leukocytes with access to host lymphoid organs (solid lines) and the number of antidonor T cells induced at these lymphoid sites (dotted lines). Stabilizing factors (asterisks) may include special cells (for example, T regulatory, suppressor), cytokine profiles, enhancing antibodies, graft secretions, and endogenous or exogenous cytoprotective molecules. How continuous immunosuppression tilts the balance against donor-specific T cells is depicted by small down-directed open arrows. GVH, graft versus host; HVG, host versus graft.