Quick Overview: Xenotransplantation
Here are some basic facts you need to understand before you can begin to talk sensibly about xenotransplantation. Don't memorize this list. Just read through it. After reading the articles in this issue of Snapshots, you will (we hope) be able to say, "OK, I knew that," about each point.

  • An organ or tissue transplanted from a member of one species (such as a pig, baboon, or chimp) into another (such as a human) is a xenotransplant. An organ or tissue transplanted between two members of the same species (such as two humans) is an allotransplant. (Xeno is Greek for foreign; allo is Greek for different).
     
  • Pathogens are microscopic creatures that make you sick. These include both bacteria and viruses.
     
  • Bacteria are single-celled microorganisms that multiply on their own.
     
  • Viruses are bits of DNA or RNA surrounded by a protein coat. They must take over a host cell's replication machinery in order to multiply.
     
  • Your immune system attacks foreign pathogens without mercy. This is great for protecting you from infection and sickness. This is not good if you get a transplanted organ. Your immune system sees the transplant as a foreign invader and tries to kill it. This is called transplant rejection.
     
  • The more closely related the transplant donor is to you, the transplant recipient, the less violently your immune system reacts.
     
  • If the donor is your identical twin, your immune system won't attack the transplant at all. If the donor is not your identical twin, your immune system will attack the transplant to some extent.Some people are good transplant matches, and rejection hardly occurs. Others are bad matches, and rejection is very vigorous.
     
  • If the donor is from a completely different species (such as a pig or a chimp) your immune system goes all out to kill the transplant. The more distant the species, the more vigorous the attack.
     
  • For human-to-human transplants, doctors have become very good at using drugs to suppress the immune system just enough to prevent rejection without leaving people completely open to attack by pathogens. Most transplant recipients have to take antirejection drugs for the rest of their lives.
     
  • More than 20,000 human-to-human transplants are performed each year in the United States. Most transplant recipients would soon die without the transplant.
     
  • Donor organs mostly come from people who have died in accidents or violence. Either donors agreed ahead of time to donate their organs when they die, or relatives agree to donate their loved one's organs around the time of death.
     
  • There are not enough donor organs to fill the need. 50,000 people are on waiting lists for donor organs. About 4,000 of these people die from their illnesses each year, still waiting.
     
  • Xenotransplantation is still very experimental, but many researchers have high hopes that the scientific problems posed by rejection can be solved.