The immune system is designed to attack foreign bodies, and the unrecognized tissue of an organ transplant is no exception. Drugs that suppress the immune system are used to prevent organ rejection, but they leave patients susceptible(感染しやすい)to infection(伝染病). Excitingly, a 15-year-old Australian girl has become the world's first known transplant patient to change blood groups(血液型)and take on the immune system of her organ donor. Doctors suspect that a post-operative infection may have allowed stem cells from the donor liver to migrate into Demi-Lee Brennan's bone marrow(骨髄)and proliferate(繁殖する). If doctors learn to manipulate(うまく処理する)this process, they will be able to improve transplant outcomes for thousands of patients.