IL15 and infection: Stimulation by infections, cytokines [e.g., type I interferon (IFN), interleukin-15 (IL-15), IL-12, IL-18, IL-21 and IL-1β; either alone or in combinations], and pathogen-associated molecular patterns (PAMPs) can prime NK cells by lowering the threshold for further activation (43), and by inducing expression of effector molecules (44, 45).