Stem cells are immature blood cells that mature into red blood cells, white blood cells, and platelets. The mature cells carry oxygen to tissues, defend against infection, and prevent bleeding. Life-threatening problems result if patients do not produce stem cells or if powerful cancer treatments or immune body processes destroy stem cells. In either instance, doctors can replace stem cells and restore the body's ability to produce the blood cells that the patient needs. One method that doctors may use is allogeneic stem-cell transplantation (ASCT). In this type of transplantation, patients receive healthy stem cells from another person.