The newly transplanted cells home to bone marrow, engraft, then begin to produce healthy new blood cells. Peripheral blood stem cells generally restore the bone marrow within about two weeks, but it can take up to five weeks if the stem cells come from the marrow itself. Restoring complete immune function can take several months in autologous transplants, and one to two years in allogeneic transplants. During the patient's recovery, doctors can determine whether the cancer has returned by taking blood samples or aspirating small amounts of bone marrow through a needle for biopsy.