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.