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  3. The first official transplant from an animal to a human occurred back in 1682. The recipient was a Russian nobleman with an inju

The first official transplant from an animal to a human occurred back in 1682. The recipient was a Russian nobleman with an inju

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问题     The first official transplant from an animal to a human occurred back in 1682. The recipient was a Russian nobleman with an injured head; a surgeon patched him up with a piece of dog skull. By the late 1800s frog skin grafts were the fashion. One British army surgeon claimed hundreds of successes for severe burns victims and patients with skin ulcers. Xenotransplantation, as such animal-to-human donations are known, has had a surprisingly long history. Unfortunately almost all attempts have been miserable failures.
    Science only learnt why in the 1940s with the discovery of rejection. When human organs are transferred from person to person, the immune system attacks and destroys what it sees as a foreign substance. When the body part comes from another species the attack is usually even more fierce. The fact is, now there aren’t enough human donors to satisfy the demand and there will never be. So researchers the world over are seeking ways to get around the rejection reactions for xenotransplants. Many are confident we will eventually perfect the animal-human donation and the organ shortage will be solved for ever.
    The most likely candidates are pigs. They are roughly the same size as people, so mechanically speaking, their organs should be up to the job. Pigs also have ethical advantages. It could be said, for instance, that chimpanzees should never be used for human transplants; that there is something inherently wrong with killing and harvesting the body parts of a perfectly healthy ape just to keep some elderly, sick person alive. It’s hard to make the same argument for pigs: We breed them to eat anyway. Still, there will no doubt be people who feel uneasy about having parts of other species put into people. Personally, the alternative—death—makes me more uneasy.
    But there’s a very real concern with xenotransplantation: the possibility of new diseases being introduced in the process. On the positive side, people and pigs have been living in close contact for thousands of years and only a few illnesses have been passed on to us. Some say therefore the chance of a new highly deadly malady being caught is remote. But there’s a big difference between living with a pig and having part of one inside you. Proviruses are a potential source of problems. When certain viruses infect animals or people they hijack the victim’s body and direct it to produce more copies of the infectious virus. What is worse, the viruses could be passed from generation to generation. Patients undergoing xenotransplantation will be especially vulnerable, because their natural defenses are down. The hunt is on to find any potentially dangerous pig proviruses.  
Almost all attempts in xenotransplantation have failed because
选项 A、rejection was discovered.
B、the organs from other animals carried certain viruses.
C、they were unethical practices and were objected to by the society.
D、the recipient’s body mechanisms rejected a foreign substance.
答案D
解析 几乎所有的异种器官移植尝试都以失败告终，这是因为[A]人们发现了排异反应。[B]其他动物的器官携带某些病毒。[C]这些尝试都是不道德的，遭到全社会的反对。[D]受体的身体结构排斥外来物质。文章第二段指出，直到20世纪40年代发现了排异反应，人们才明白导致失败的原因。当人类器官从一个人移植到另外一个人时，受体的免疫系统将其视为外来物质进行攻击和破坏。如果供体是另一物种，则排异反应通常更为强烈。因此，正确答案为[D]，原因在于受体排斥外来物质。
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