THE INNATE AND ADAPTIVE IMMUNE SYSTEMS: DEFINITIONS, CONTEXT, AND CONTRASTS
Standard accounts of the immune system emphasize the antigen-specific immunity and memory afforded by the adaptive immune system, contrasting it with the “nonspecific” defenses provided by the phylogenetically more ancient innate immune system. While study of the innate immune system has undergone a recent renaissance, most immunology textbooks still present innate immunity as an initial stopgap defense that holds the line until the “real” (efficient, effective, sophisticated) adaptive immune system can take over. There are obvious flaws in such formulations. First, while adaptive immunity may usefully be seen as a single system – with its cells (B and T lymphocytes) and antigen receptors (immunoglobulins [Ig], T-cell receptors) depending directly on the evolution of the recombination-activating gene (RAG) in jawed vertebrates – innate immunity, present in all metazoans, is a congeries of pathways. “Innate immune systems” is a much better term. Second, the innate immune systems are in no way less sophisticated than the adaptive immune system, having been under evolutionary pressure for far longer. Third, the innate immune systems are not of secondary importance; the adaptive immune system is directly dependent on the former for efficient and appropriate activation. Fourth, innate immune effector mechanisms are not less effective than adaptive immune effector mechanisms.