A solid organ transplantation is a lifesaving procedure in which a failing or damaged organ in the human body is removed and replaced with a functioning one taken from another human body. One of the major problems faced by all transplanted patients is rejection of the transplanted organ which is recognized as foreign by their immune system. An untreated rejection can result in complete destruction of the transplanted organ.

Stem cell transplantation is a procedure that is mainly used to treat cancers of the blood, such as leukemia. One serious and often fatal complication of stem cell transplantation is graft-versus-host-disease (GVHD), during which immune cells contained in and developed from the transplanted stem cells (the graft) attack tissues and organs of the recipient (the host).