Sir Peter Brian Medwar was a Brazilian born British zoologist who was awarded the Nobel Prize for Physiology and Medicine for discovering the theory of ‘acquired immunological tolerance’. He received this prize in 1960 along with Sir Frank Macfarlane Burnet. His development of the theory helped to find a method by which tissue and organ transplantation became possible later. The theory first suggested by Sir Frank was that soon after birth all vertebrates develop the capability to differentiate between elements that are a part of the body and those that are foreign. This theory was supported by Medawar though it was in contradiction to the earlier theory that vertebrates had this ability right from the conception stage itself. He proved the fact that skin grafts from one twin were acceptable by the other as both the twins had the same antigens which were required for the development of immunity. This discovery changed the basic idea of immunology and suggested that a fully developed immunity system can be altered so that it tries to suppress any rejection of foreign organs and tissues. His research work during the Second World War in the field of rejection of skin grafts for an increasing number of burn victims caused by German bombings became a very vital issue.