Organ transplantation is a routine, life-saving procedure, but demand for organs far outstrips supply.

When organs such as the heart, kidney or liver fail, and no drugs are available, a transplant can be the only remaining option.

Organ transplants were first attempted a hundred years ago, but early efforts failed because of rejection - the body sees the new tissue as 'foreign' and the immune system destroys it.

The modern era of transplantation was ushered in by improved tissue matching and the development of immunosuppressants - drugs that turn off the immune response. The transplanted organ is not rejected, though patients remain vulnerable to infections and must take medication throughout their lives.

During the 1960s, patients rarely survived for long. Gradually, though, immunosuppressants and surgical techniques improved and organ transplantation is now relatively straightforward. People who receive a new organ can expect to survive and enjoy good quality of life.

Some organs - such as kidneys and lungs - can be provided by living donors. Most people can survive with one kidney or without part of their liver (which can also regenerate).

Early experiments explored the potential of animal as well as human organs. However, animal organs are rejected even more violently than human tissue, and the immunosuppressants that enable human organs to survive cannot protect animal organs from destruction.

Transplantation would be more common if more donor organs were available. With fewer people being killed on the road, and other trends, there are not enough organs for the people who need them. More than 9000 people in the UK need an organ transplant, and more than 400 die every year while on the transplant waiting list.

Ways to overcome this shortfall have been proposed, such as switching from an 'opt-in' to an 'opt-out' system, where organs will be used unless someone specifically objects. A survey in 2003 revealed that 90 per cent of people supported organ donation but only around a quarter were actually on the donor register. Meanwhile, researchers are working on alternatives to transplantation.