The immune system performs two vital functions that are critical for the maintenance of homeostasis and survival:

(1) inducing an effective and safe response against foreign antigens (infectious and noninfectious) and (2) avoiding a response to components of “self” antigens by enforcing stringent regulatory controls over dangerous self- reactive cells that are capable of mounting devastating immune attacks on “self” tissues. Because the induction of immune responses depends on antigens, this chapter first discusses the nature and characteristics of antigens.