Dr. Stephen Black looks at The Problem Behind the transplants Produced by PHILIP DALY Identical twins apart, every individual is different. In each of us there is a defence system which constantly distinguishes 'self' from ' non-self.' Every foreign invader is rejected, whether it be a deadly germ or someone else's heart. Some mothers even reject their own babies; and even the familiar allergies, asthma and hay fever can be caused by ' 'foreign' pollens in the air we breathe. The immunologists have made use of this reaction to protect us from disease, as every mother knows when she has her baby immunised. There is now evidence that the body may also try to reject its own cancer cells as if they belonged to somebody else, and research in immunology may, for the first time, have provided us with the first real clue as to where the key to the cancer problem might lie. In this programme some of the world's leading immunologists, whose previous work has already enabled surgeons to transplant hearts, kidneys, and livers between different individuals, talk about their present research and some of the promises it holds.