Reducing rejection: Foster familiarity Organ plus donor bone marrow stem cells: The patient gets a donor kidney plus radiation (which kills some of the patient’s bone marrow cells to make room for the donor’s), followed by a bone marrow transplant. Those facilitator cells help the donor marrow settle in, so much so that more than 95 percent of the immune cells in the recipient’s blood are made by the donor stem cells. Immune cells called B cells are collected from the donor’s blood or spleen and added to the dish to train the T regs to become familiar with the donor organ so they will quash immune attacks against the organ but not interfere with fighting off infections or cancer.