“Allo” in allogeneic means “other” which implies that the stem cells come from another person (donor) whose genetic typing matches yours. The donor is usually directly related to you such as your brother or sister. Special blood tests are performed to check for donor/recipient compatibility.
In some cases, if you don’t have a good match in your family, compatible donors may be found through the international bone marrow registries. Also, blood from the umbilical cords of newborns have been used as a source of stem cells and used mainly for treatment of children (as the number of stem cells in this blood is not enough to treat adults).