All organisms are connected in a complex web of relationships. Although many of these are benign, not all are, and everything alive devotes significant resources to identifying and neutralizing threats from other species. From bacteria through to primates, the presence of some kind of effective immune system has gone hand in hand with evolutionary success. This article focuses on mammalian immunity, the challenges that it faces, the mechanisms by which these are addressed, and the consequences that arise when it malfunctions.