Beyond the most primitive of oligocellular creatures, all animals possess the capacity to defend themselves against invading pathogens. In vertebrates, defense against pathogens is accomplished by two independent, but complementary, response systems: innate immunity and adaptive immunity. Innate immunity is thought to be phylogenetically older and is comprised of a set of cells and molecules which, in a stereotypic manner detect, respond to, and, often, eliminate pathogens. By contrast, adaptive immunity is comprised of cells (lymphocytes) that throughout life generate unique receptor molecules that recognize with extraordinary specificity molecules expressed by invading pathogens. In advanced vertebrates, the innate and adaptive immune systems interact coordinately in responding and destroying invading pathogens.