Xenograft
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A xenograft (xenotransplant) is a transplant of tissue from a donor of one species to a recipient of another species. The terms heterograft and heterotransplant are also sometimes used, while the term homograft refers to a same-species transplant.
Cross-species transplants are more likely to produce host-vs-graft or graft-vs-host reactions than same-species transplants, because of the lack of antigenic similarity. Organisms which have been genetically engineered to reduce this lack of similarity have been produced but are not yet used to any significant degree in medical care.
The first xenografts involving human beings were performed by Dr. Keith Reemtsma between 1963 and 1964, in which thirteen chimpanzee kidneys were transplanted into humans. Twelve of the thirteen recipients died within two months, while the thirteenth survived for nine months after the procedure.
The first cross-species heart transplant was performed in 1964, in which a 68-year-old man received a chimpanzee heart. He survived only two hours. Several other attempts have been made to transplant primate hearts into humans, with no patients surviving more than twenty-one days.
While whole-organ xenografts have thus far been unsuccessful, less radical transplants have demonstrated great success. Hundreds of thousands of patients have received pig heart valves since 1975, when the procedure first became commercially available. Cow heart valves have likewise been used in humans since 1981.
Xenografts have been a controversial procedure since they were first attempted. Many, including animal rights groups, strongly oppose killing animals in order to harvest their organs for human use. Legitimate medical concerns exist about possible disease transfer between animals and humans, such as the porcine endogenous retrovirus found in pig tissues. Religious beliefs, such as the Jewish and Muslim prohibition against eating pork, may also present concerns for some.
In general, however, the use of pig and cow tissue in humans has met with little resistance. The tissue is harvested from agricultural animals that were already being butchered, which is less offensive to most people than the idea of raising a primate solely as an organ donor. Similarly, while some individual Jews may not wish to receive a pig valve based on their personal beliefs, the rabbinical view is that the use of pig valves in humans is not a violation of kashrut law.
A continuing concern is that cows and pigs have different lifespans than humans and their tissues age at a different rate.