Any reaction by the immune system.
(Science: immunology) alteration in the reactivity of an organisms immune system in response to an antigen, in vertebrates, this may involve antibody production, induction of cell-mediated immunity, complement activation or development of immunological tolerance. a bodily defense reaction that recognizes an invading substance (an antigen: such as a virus or fungus or bacteria or transplanted organ) and produces antibodies specific against that antigen.An immune system response due to the presence of a particular foreign antigen, such as bacteria, fungi or virus.