Immunity (Latin: immunitas, “freedom from”) describes a state of adequate defense against infection, i.e., bodily invasion by microorganisms. Immunity is established by the activities of the immune system (see entry on “ Immune Function”). Immunity also used to describe a state of adequate defense against neoplasms (cancers) insofar the immune system is involved. The entry on “ Immune Function” provides further details on the main types of immunity (innate and adaptive) and mechanisms by which the immune system establishes a state of immunity. Further information can be found in Abbas, Lichtman, and Pillai (2012) and Murphy (2011).