Derived from Roman law, for more than two thousand years, immunity served almost exclusively as a juridico-political concept that modulated the criteria of inclusion and exclusion determining how citizens became subjects of European nation-states. However, once introduced into bio-science in the wake of—and as an explanation for—Pasteur's vaccination experiments, biological "immunity" began to be used to describe a "natural" and necessary function of complex organisms. Today when we use the metaphor immunity to refer to organismic "self-defense," we usually ignore the political and legal history that the concept necessarily incorporates. By first considering the creative role that metaphor plays in bio-science generally and then specifically considering the historical residue that the concept of immunity has accreted throughout its long history, this essay suggests that the biological phenomena currently encompassed by the term "immunity" may exceed, or even contradict, the metaphor that we use to make sense of them.