The discovery of acquired immunological tolerance
Burnet developed the concept that the immune system develops predicicted that it would be experimentally possible to fool a body into believing that a foreign substance was 'self'.
These experiments were carried out by Medawar, who performed tissue grafts on twin calves and in mice. The large number of inbred mice available was an advantage, because there was little genetic variation between mice from the same litter. Foreign tissue was introduced into mouse embryos whilst still in the womb, and the young mice were then allowed to develop normally. When tissue grafts were performed on these mice, they had no immune reaction if the tissue was the same type that had been introduced in the womb. However, they reacted strongly to the grafting of other foreign tissue.
The mice were said to have 'aquired immunological tolerance' to a particular tissue.