Immunologic Aspects of Organ Transplantation

Susan Smith MN, PhD


June 17, 2002

Isograft, Allograft, or Xenograft?

Organ transplantation is the surgical replacement of diseased organs with healthy organs (grafts) from live or cadaver donors. The genetic relationship between the donor and the recipient is fundamental to all else in transplantation. Successful transplantation between 2 individuals who are not genetically identical requires diligent assessment of those differences before transplantation, and individualized immunosuppressive therapy after transplantation, to minimize recognition and subsequent rejection of the foreign graft by the recipient's immune system.

Except in isolated experimental cases, organs used in clinical transplantation are either isografts or allografts. An isograft is an organ transplanted from a donor who is genetically identical to the recipient (ie, identical twins). Isografts are also called isogeneic and syngeneic grafts.

An allograft is an organ transplanted from a donor to a recipient of the same species who is not genetically identical. Allografts are also called allogeneic grafts and homografts. Although isografts would be preferable to allografts, for obvious reasons the majority of organs transplanted are allografts. Allografts can be donated by a person who has died (cadaveric allograft), a living-related individual (eg, a parent or a sibling), or a living-unrelated individual. A xenograft is an organ transplanted from a donor to a recipient of a different species (eg, baboon to human). Xenografting is examined in detail in Chapter 6, Xenotransplantation.