- Stem cells are immature blood cells that grow in the bone marrow and produce all of the cells needed for normal blood and immunity. Stem cells can be taken from one person (donor) and given to another person (recipient) through allogeneic stem cell transplantation. Donor stem cells can then replace the recipients stem cells in the bone marrow, restoring normal blood production and immunity. Most allogeneic transplants now use stem cells collected from the donors blood in a process called peripheral blood stem cell transplantation.