<qa>  What is the immune system damaged?People who have had an allogeneic hematopoietic stem cell transplant HCT have bone marrow or an immune system that is damaged. They get stem cells from a donor who is a relative. Researchers want to study stem cell donors and recipients to learn about the long-term effects of HCT. They want to learn how the stem cells change and how to improve their ability to fight cancer. 