| Throughout the world there is a
shortage of human organs for transplants. In Britain, for example, 6000 people
are waiting for organs ——5000 for kidneys, and the {{U}}(51) {{/U}} for
hearts, lungs and livers. {{U}}(52) {{/U}}, only about 1750 kidney, 500
heart and lung, and 650 liver transplants are {{U}}(53) {{/U}} each
year. And the waiting {{U}}(54) {{/U}} increases at five percent a year.
In the United States only half of the 30000 in need of organs {{U}}(55)
{{/U}} them. Hence the great interest in animal-to-human transplants, known as xenografts or xenotransplants (from the Greek work xenos, meaning strange of foreign) {{U}}(56) {{/U}} most scientists believe is the only long-term solution to the organ {{U}}(57) {{/U}}. There have been attempts at xenotransplants {{U}}(58) {{/U}} the beginning of this century but neither has been successful; the longest {{U}}(59) {{/U}} was a 20-day-old baby called Fae, who in 1984 was {{U}}(60) {{/U}} the heart of a baboon. The main {{U}}(61) {{/U}} with any organ transplant is that the immune system of the patient receiving the organ sees the transplant organ {{U}}(62) {{/U}} an invader. The immune system therefore launches a massive attack on the invader, activating enzymes knows as complement, which attack the {{U}}(63) {{/U}} body, eventually killing the patient as well. This rejection has been {{U}}(64) {{/U}} when transplanting human organs between humans by the close matching of tissue and the long-term use of drugs known as immuno-suppressants, first introduced just over ten years ago. With xenotranplants, however, rejection is even more {{U}}(65) {{/U}}; a normal pig's heart, for example, transfuses with human blood can be destroyed in 15 minutes. |