IL2 and infection: For example, during infection with two classic Th1-inducing pathogens, Salmonella enterica and Plasmodium falciparum, NK cells require activating signals from three distinct sources: a priming or pre-activation signal of either IL-2 or IL-15 from T cells or dendritic cells (DCs) respectively, IL-12 and IL-18 from macrophages, and direct contact with macrophages (41, 42).