The Collaborative International Dictionary of English v.0.48:

immune system \im*mune" sys"tem\, n. (Biol.)
   The complex of cells, cellular processes, and substances
   within and diffused throughout an organism which allow the
   organism to counteract or destroy noxious foreign substances
   introduced into the body, destroy infectious agents such as
   bacteria and viruses, destroy malignant cells, and remove
   cellular debris, thus protecting the organism against many of
   the potentially harmful external agents and internal events
   that could lead to sickness or death. The system has numerous
   interacting components, including circulating antibodies,
   antibody-producing cells, white blood cells and lymphokines,
   lymph tissue and lymph nodes, and stem cells which may
   differentiate into other types of cell, together with the
   thymus and spleen. The system is responsible for the
   phenomenon of immunity[3]. See also immunoglobulin and
   antibody.
   [PJC]

WordNet (r) 3.0 (2006):

immune system
    n 1: a system (including the thymus and bone marrow and lymphoid
         tissues) that protects the body from foreign substances and
         pathogenic organisms by producing the immune response