(1) Free from the possibility of acquiring a given infectious disease; resistant to an infectious disease. (2) Pertaining to the mechanism of sensitization in which the reactivity is so altered by previous contact with an antigen that the responsive tissues respond quickly on subsequent contact, or to in vitro reactions with antibody-containing serum from such sensitized people. [L. immunis, free from service, fr. in, neg., + munus (muner-), service]