CD4 and infection: Although CD4+ T cells are dispensable for the generation of primary effector CD8+ T cell responses to some infections, CD27 on CD8+ T cells interacting with CD70 on APCs primed by CD4+ T cell “help” via CD40(APC)/CD40L(CD4+ T cell) activation is required for the generation of functional memory CD8+ T cells marked by reduced proliferative capacity during recall responses (26).