Donor-specific blood transfusion is known to induce alloresponses and lead to immunosuppression. We examined their underlying mechanisms by employing fully allogeneic rat combinations. Transfused recipients efficiently produced alloantibodies of the IgM and IgG subclasses directed against donor class I MHC. The recipients exhibited active expansion of CD4+ T-cells and CD4+FOXP3+ Treg cells, followed by CD45R+ B-cells and IgM+ or IgG subclass+ antibody-forming cells mainly in the spleen. From 1.5 days, the resident MHCII+CD103+ dendritic cells in the splenic T-cell area, periarterial lymphocyte sheath, formed clusters with recipient BrdU+ or 5-ethynyl-2'-deoxyuridine+ cells, from which the proliferative response of CD4+ T-cells originated peaking at 3-4 days. Transfusion-induced antibodies had donor passenger cell-depleting activity in vitro and in vivo and could suppress acute GvH disease caused by donor T-cells. Furthermore, Treg cells significantly suppressed mixed leukocyte reactions in a donor-specific manner. In conclusion, single blood transfusion efficiently induced a helper T-cell dependent anti-donor class I MHC antibody forming cell response with Ig class switching, and a donor-specific Treg cell response mainly in the spleen, probably by way of the indirect allorecognition via resident dendritic cells. These antibodies and Treg cells may be involved at least partly, in the donor-specific transfusion-induced suppression of allograft rejection.