 The basic schema of vertebrate immunity consists of two evolutionary distinct ways to perceive the organism’s external environment and to monitor its inner state. The first modality, more primitive and less refined in its perceptive mechanisms, is referred to as the “innate” or “natural” immune system. This arm of immunity is composed of a variety of primordial phagocytes—“eating cells”—possessing distinctive characteristics based on their tissue distribution. They ingest particulate microbes and destroy effete, cancerous, and damaged host cells. These immunocytes constitute the first line of the immune encounter and comprise the “resting” state of physiological immunity as they “police” on-going processes of surveillance and restoration. This system has autonomous functions, but the incorporation of antigen (a substance that stimulates an immune response) may also lead to integration with the second mode of immunity, the so called, “acquired” immune system. This second arm of immunity has evolved mechanisms that are highly specific in their recognition capabilities and exhibit immune “memory” of previous sensitization. Acquired immunity is mediated by several classes of lymphocytes that may be stimulated by phagocyte first-encounters or as primary responders under certain pathological conditions. The B-lymphocytes produce antibody that recognizes antigens by structural homology and, correspondingly, T-cells have specific antigen-matching receptors on their cell surface. The initial recognition event leads to activation of a variety of effector reactions that destroy the immune target. However, the decision not to respond is also a recognition event and results in immune “tolerance”. Both the innate and acquired immune cellular elements are regulated by a vast array of soluble molecular factors that serve to either activate or dampen immune responses. The complexity of diverse cell types, the mediators of their interactions, the contextual determinants that determine immune reactivity, and the wide range of physiological functions in which immunity participates, has defied comprehensive modeling. Consequently, immunology offers a fecund example of biological organizational complexity, which, in turn, leads to the fundamental philosophical question: On what basis is organismal identity established and maintained?