Miraculously, transplanted stem cells find their way to the bone marrow missing the stem cells. How do they achieve this?
The transplantation is an equivalent of a process that naturally occurs in human body, homing of the hematopoietic stem cells.
Immature hematopoietic stem cells have the ability to pass the bone marrow barrier, and therefore is able to migrate between bones and other organs within an individual. (e.g. thymus, which is how it can produce T cells.) Williams et al. believe that the process, homing, is a complex interaction and is similar to the migration of leukocytes to inflammatory sites. 1
Peripheral stem cell transplantation makes use of this ability. The donor first will be administered with certain drugs to proliferate stem cells in bone marrow and hereby raising the concretion of hematopoietic stem cells in blood. Those stem cells in blood then will be collected and transfused into the recipient. Eventually those cells will find their way home.