The Collaborative International Dictionary of English v.0.48:

immunology \immunology\ n.
   The science which studies the immune system, the processes of
   immunity, and the nature of the immune response, and
   techniques of analysis which use the immune response.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

immunology
    n 1: the branch of medical science that studies the body's
         immune system