Pretransplant blood transfusions have been shown to improve organ allograft survival. However, the immunologic mechanism leading to this beneficial effect of blood transfusions is still unknown. The observation that transfusions sharing at least one HLA-DR antigen (human leukocyte antigen) with the recipient are more effective than HLA-mismatched transfusions has led to the hypothesis that CD(4+) regulatory T cells are induced that recognize allopeptides of the blood transfusion donor in the context of the self-HLA-DR molecule on the donor cells. In vitro studies showed that CD(4+) T cells recognizing an allopeptide in the context of self-HLA-DR are indeed able to decrease the alloimmune response of autologous T cells by affecting the activated T cells directly or indirectly by their modulatory effect on dendritic cells. The first studies in a patient with a well-functioning kidney graft after receiving an HLA-DR-matched pretransplant blood transfusion showed that the low organ donor-specific cytotoxic T-lymphocyte response after transplantation was indeed attributable to the activity of regulatory CD(4+) T cells.