Transplantation (medicine)

Transplantation is a medical procedure in which an organ, tissue, or cells are removed from one organism and placed in a recipient organism, often to replace a damaged or missing function. The donor and recipient may be at the same location, or organs may be transported from a donor site to another location. Organs and tissues that are transplanted within the same person's body are called autografts. Transplants that are recently performed between two subjects of the same species are called allografts. Allografts can either be from a living or cadaveric source. Organs that have been successfully transplanted include the heart, kidneys, liver, lungs, pancreas, intestine, thymus and uterus. Tissues include bones, tendons (both referred to as musculoskeletal grafts), corneae, skin, heart valves, nerves and veins. Worldwide, kidneys remain the most commonly transplanted organs, followed by liver and heart. Global analyses indicate that available transplants meet a minority of worldwide need, with the Transplant Observatory reporting substantial gaps between demand and supply and year-to-year increases in registered transplant activity in recent data releases. J. Hartwell Harrison performed the first organ removal for transplant in 1954 as part of the first kidney transplant. Cornea and musculoskeletal grafts are the most commonly transplanted tissues; these outnumber organ transplants by more than tenfold. Organ donors may be living individuals, or deceased due to 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 (with the exception of corneas) can be preserved and stored—also known as "banked"—for up to five years. Transplantation raises a number of bioethical issues, including the definition of death, when and how consent should be given for an organ to be transplanted, and payment for organs for transplantation. Other ethical issues include transplantation tourism (medical tourism) and more broadly the socio-economic context in which organ procurement or transplantation may occur. A particular problem is organ trafficking. There is also the ethical issue of not holding out false hope to patients. Transplantation medicine is one of the most challenging and complex areas of modern medicine. Some of the key areas for medical management are the problems of transplant rejection, during which the body has an immune response to the transplanted organ, possibly leading to transplant failure and the need to immediately remove the organ from the recipient. When possible, transplant rejection can be reduced through serotyping to determine the most appropriate donor-recipient match and through the use of 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 global shortages, with only 10-20% of needs met, and inequities in access. Recent advances include xenotransplantation (e.g., clinical trials and case reports of genetically modified porcine organs implanted into humans that 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 preservation intervals and improved graft assessment prior to transplant.

Types of transplant

Autograft

Autografts are the transplant of tissue to the same person (e.g., kidney for nutcracker syndrome or ex vivo liver resection for tumors) to minimize rejection. Sometimes this is done with surplus tissue, tissue that can regenerate, or tissues more desperately needed elsewhere (examples include skin grafts, vein extraction for CABG, etc.). Sometimes an autograft is done to remove the tissue and then treat it or the person before returning it (examples include stem cell autograft and storing blood in advance of surgery). Autografts involve transplanting tissue within the same individual, such as skin for burns or veins for bypass surgery; rotationplasty relocates a foot/ankle to replace a knee in cancer cases. No immunosuppression needed due to genetic identity.

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. Due to the genetic difference between the organ and the recipient, the recipient's immune system will identify the organ as foreign and attempt to destroy it, causing transplant rejection. The risk of transplant rejection can be estimated by measuring the panel-reactive antibody level.

Isograft An isograft is a subset of allograft in which organs or tissues are transplanted from a donor to a genetically identical recipient (such as an identical twin). Isografts are differentiated from other types of transplants because while they are anatomically identical to 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 quite common and successful. Another example is attempted piscine–primate (fish to non-human primate) transplant of pancreatic islets. The latter research study was intended to pave the way for potential human use if successful. However, xenotransplantation is often an extremely dangerous type of transplant because of the increased risk of non-functional compatibility, rejection, and disease carried in the tissue. In the opposite direction, attempts are being made to devise a way to transplant human fetal hearts and kidneys into animals for future transplantation into human patients to address the shortage of donor organs.

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