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 is 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 et al. (2012) and Murphy (2011).