Niels Kaj Jerne, FRS (23 December 1911 – 7 October 1994) was a Danish immunologist. He shared the Nobel Prize in Physiology or Medicine in 1984 with Georges J. F. Köhler and César Milstein "for theories concerning the specificity in development and control of the immune system and the discovery of the principle for production of monoclonal antibodies". Jerne is known for three significant ideas. Firstly, instead of the body producing antibodies in response to an antigen, Jerne postulated that the immune system already has the specific antibodies it needs to fight antigens. Secondly, it was known that the immune system learns to be tolerant to the individual's own self. Jerne postulated that this learning takes place in the thymus. Thirdly, it was known that T cells and B cells communicate with each other. Jerne's network theory proposed that the active sites of antibodies are attracted to both specific antigens (idiotypes) and to other antibodies that bind to the same site. The antibodies are in balance, until an antigen disturbs the balance, stimulating an immune reaction. Early years His ancestors had lived on the small Danish island of Fanø for centuries, but, in 1910, his parents moved to London where Jerne was born in 1911. During the First World War his parents moved to the Netherlands and Jerne spent his youth in Rotterdam. After studying physics for two years at the Leiden University, Jerne moved to Copenhagen and changed his studies to the field of medicine. He graduated from the University of Copenhagen with a degree in medicine in 1947. Four years later, he was awarded the doctorate for his thesis, A Study of Avidity Based on Rabbit Skin Responses to Diphtheria Toxin-Antitoxin Mixtures. Research positions From 1943 to 1956 Jerne was a research worker at the Danish National Serum Institute and during this time he formulated a theory on antibody formation. It is said that Jerne got his revolutionary scientific idea while bicycling across the Langebro bridge in Copenhagen on his way home from work. The antibody formation theory gave Jerne international recognition and in 1956 Jerne went to work for the World Health Organization in Geneva, where he served as the Head of the Sections of Biological Standards and of Immunology. He held this post for six years until moving to the United States and the University of Pittsburgh in 1962 to work as Professor of Microbiology and