Dendritic cells (DC) are the most potent antigen-presenting cells (APC) and are able to modulate immune responses. Investigators are studying methods to exploit the immunogenic and tolerogenic properties of DC. In the context of hematopoietic cell transplantation, DC might be helpful to facilitate engraftment and prevent graft-versus-host disease (GVHD) reactions. In this paper, we review circumstantial evidence that immature plasmacytoid DC might affect immune responses after transplantation of hematopoietic cells from allogeneic donors.