Compositions and methods for reducing or preventing graft-versus-host disease associated with allogeneic stem cell transplantation are described. A method includes harvesting hematopoietic cells from a patient with an immunologic disorder and harvesting hematopoietic cells from a donor. The harvested hemaptopoietic cells from the patient and the donor are mixed ex vivo for a period of time sufficient to activate lymphocytes within the hematopoietic cells harvested from the donor such that an immune reaction occurs and an activated portion and an unactivated portion is produced. A rhodamine B derivative is then added to the mixture of cells and the mixture is irradiated at a suitable wavelength and intensity for the selective destruction and/or inactivation of the activated portion without substantially affecting the unactivated portion or causing systemic toxicity in said patient. The irradiated cells are then infused into the patient.
