Sentence:  Stem cells from bone marrow that can give rise to new immune cells are taken out from a patient, and then physicians use drugs to purposely obliterate the patient's entire immune system, a strategy called immunoablation. After that, the <WSD>preharvested</WSD> stem cells are infused back into the patient, where they can create fresh bone marrow and, it is hoped, a new functioning immune system. 
Meaning of the word marked with <WSD> tags:  Harvested in advance