Transplantation (medicine)

Transplantation is a surgical procedure where an organ, tissue, or cells are taken from one organism and put into a recipient, usually to take over a function that is damaged or missing. The donor and recipient might be in the same place, or organs can be moved from the donor site to another location. When tissue is moved from one spot to another within the same person, it is called an autograft. Transplants between two individuals of the same species are known as allografts, and these can come from either a living donor or a deceased one.

Organs that have been successfully transplanted include the heart, kidneys, liver, lungs, pancreas, intestine, thymus, and uterus. Tissues that can be transplanted include bones, tendons (together called musculoskeletal grafts), corneas, skin, heart valves, nerves, and veins. Worldwide, kidneys are the most commonly transplanted organs, followed by the liver and then the heart. Global data shows that available transplants meet only a small fraction of the worldwide need; the Transplant Observatory reports large gaps between demand and supply, though recent data shows year-to-year increases in registered transplant activity. The first organ removal for transplant was performed by J. Hartwell Harrison in 1954 as part of the first kidney transplant. Cornea and musculoskeletal grafts are the most common tissue transplants, outnumbering organ transplants by more than ten times.

Organ donors can be living people, or deceased individuals who have died from either brain death or circulatory death. Tissues can be recovered from donors who have died from circulatory or brain death within 24 hours after cardiac arrest. Unlike organs, most tissues—except corneas—can be preserved and stored, or "banked," for up to five years. Transplantation raises several bioethical issues, such as how death is defined, when and how consent for organ donation should be given, and whether payment for organs is acceptable. Other ethical concerns include transplantation tourism (medical tourism) and the broader socioeconomic context in which organ procurement or transplantation happens. Organ trafficking is a particular problem, as is the ethical issue of giving patients false hope.

Transplantation medicine is one of the most challenging and complex fields in modern medicine. Key areas of medical management include transplant rejection, where the body’s immune system attacks the transplanted organ, potentially causing it to fail and requiring immediate removal. When possible, rejection can be reduced by serotyping to find the best donor-recipient match and by using immunosuppressant drugs.

As of September 9, 2022, the United States reached one million cumulative transplants, according to the Organ Procurement and Transplantation Network (OPTN), with kidneys being the most common globally. Challenges include worldwide shortages—only 10-20% of the need is met—and inequities in access. Recent advances include xenotransplantation (such as clinical trials and case reports of genetically modified pig organs implanted into humans, which have provided early safety and feasibility data and identified important infectious and immunologic risks), machine perfusion, and normothermic ex-vivo perfusion systems that have extended how long organs can be preserved and improved how grafts are assessed before transplant.

**Types of transplant**

**Autograft** Autografts are transplants of tissue within the same person—for example, using a kidney for nutcracker syndrome or performing an ex vivo liver resection for tumors—to minimize rejection. Sometimes this is done with surplus tissue, tissue that can regenerate, or tissue needed more urgently elsewhere (such as skin grafts or vein extraction for coronary artery bypass grafting). Occasionally, an autograft is performed to remove tissue, treat it or the person, and then return it (examples include stem cell autografts and storing blood before surgery). Rotationplasty relocates a foot and ankle to replace a knee in cancer cases. No immunosuppression is needed because the tissue is genetically identical.

**Allograft and allotransplantation** An allograft is a transplant of an organ or tissue between two genetically non-identical members of the same species. Most human tissue and organ transplants are allografts. Because of the genetic difference between the organ and the recipient, the recipient’s immune system sees the organ as foreign and tries to destroy it, causing transplant rejection. The risk of rejection can be estimated by measuring the panel-reactive antibody level.

**Isograft** An isograft is a type of allograft where organs or tissues are transplanted from a donor to a genetically identical recipient, such as an identical twin. Isografts are different from other transplants because, while they are anatomically the same as allografts, they do not trigger an immune response.

**Xenograft and xenotransplantation** A xenograft is a transplant of organs or tissue from one species to another. An example is porcine heart valve transplant, which is common and successful. Another example is a research attempt at piscine–primate (fish to non-human primate) transplant of pancreatic islets, intended to pave the way for potential human use. However, xenotransplantation is often very dangerous due to the increased risk of poor compatibility, rejection, and diseases carried in the tissue. In the opposite direction, attempts are being made to transplant human fetal hearts and kidneys into animals for future transplantation into human patients to address the shortage of donor organs.

**Domino transplants** In people with cystic fibrosis...