Publisher Summary This chapter introduces the important concepts of the immune system, including elements that are common to the innate and adaptive immune responses and elements that distinguish them. The chapter summarizes and compares the general characteristics of innate and adaptive immune responses. Most foreign antigens are eliminated by the mechanisms of innate immunity. Only those antigens that succeed in penetrating the innate defenses evoke adaptive immune responses. Innate immunity involves both pre-existing physical barriers that show little or no pathogen specificity and induced cellular responses of broad specificity. In contrast, adaptive immune responses must be induced and require the activation of B and T lymphocytes. Each lymphocyte clone expresses cell surface antigen receptors of a single specificity, and each clone is activated only upon the interaction of these receptors with complementary antigen. Activated Tc lymphocytes differentiate into cytotoxic effectors (CTLs) capable of lysing tumor cells and cells infected with intracellular pathogens, and activated Th lymphocytes differentiate into cytokine-secreting Th effectors that support B cell and Tc cell functions.