Hello, good morning. I am a biology student interested on immunoloogy field and I found a question in a book (Kuby immunology, 7th edition) I really can't understand why it is like that. I would be grateful for your help. Clinical Focus Question: Hematopoietic stem cells (HSC) were transplanted from a woman into a genetically unrelated man whose hematopoietic system was destroyed by radiation. Some of the received HSC cells differentiated into pancreas, liver, and heart cells. T lymphocytes that resulted from the same donated HSCs do not attack the host, nor do they attack the pancreas, liver, and heart cells that resulted from the same HSCs1. I don't understand why T lymphocytes don't attack pancreas, liver and heart cells, because I understand that T cells go through a process of tolerance in the recipient's thymus, causing them to attack by not being part of the host to which currently belong. It is as if HSCs granted tolerance to developing T lymphocytes from the start, without even going through the thymus. Again, I would be really grateful for your help. PD: I'm from Colombia, so English is not my native language. I'm sorry about the mistakes I made. Owen, J. A., Punt, J., & Stranford, S. A. (2013). Kuby immunology. New York: WH Freeman.