Specialized effector activities that are required to eliminate various pathogens involve cytokines produced by specialized CD4+ T cells subsets, dogmatically termed Th1 and Th2 cells. Despite some oversimplifications, this paradigm is useful for organizi

Typical immune responses lead to the prominent clonal expansion of antigen-specific T cells followed by their differentiation into effector cells. Most effector cells die at the end of the immune response but some of the responding cells survive and form long-lived memory cells. The factors controlling the formation and survival of memory T cells are discussed. Recent evidence suggests that T memory cells arise from a subset of effector cells. The longevity of T memory cells may require continuous contact with cytokines, notably IL-15 for CD8+ cells.