Sir Peter Brian Medawar OM CBE FRS ( 28 February 1915 -- 2 October 1987 ) was a British biologist . Sir Peter Brian Medawar OM CBE FRS ( 28 February 1915 -- 2 October 1987 ) was a British biologist , whose work on graft rejection and the discovery of acquired immune tolerance was fundamental to the practice of tissue and organ transplants .