Stem cells are simple cells that are able to develop into various types of specialized cells. Stem cells reside in the bone marrow. They can also be found in large numbers in the blood if the donor receives drugs called growth factors. Stem cells can also be found in umbilical cord blood, which lowers the risk of graft-vs-host disease, but the number of stem cells found in cord blood is small.
Because bone marrow has been the original stem cell source since the 1960s, "bone marrow transplant" is the term most often used, but it is stem cells taken from the bone marrow and other blood sources that are transplanted and grow in the recipient.