Since the donor NK cells are from a haploidentical individual, they are different enough to be recognized as foreign cells and will be killed immediately ("rejected") by the patients own immune system unless the immune system is restrained. So, to allow the donor NK cells time to kill neuroblastoma cells before they are "rejected", a chemotherapy regimen is first given to the patient to temporarily restrain the patient's own immune system. This also allows "room" for the donor NK cells to live, multiply and function.