The immune response to an allergen depends on an initial sensitization process, with future exposures triggering a 2-part allergic response including an early and late phase. The process by which an allergen is recognized has become clearer. Similarly, the roles of the preformed mediators responsible for many of the signs and symptoms of the early-phase response have been elucidated. Recent work also has shed some light on the cells and mediators involved in the late-phase reactions. This article discusses some of this recent work and reviews the basics behind of all of the stages of the allergic response.