IL2 and influenza: NK cells induced by influenza vaccination share all the characteristics of cytokine preactivated memory-like (7) NK cells, including long-term enhancement (for at least a month) of IFN-γ responses to innate cytokines and MHC class I–deficient target cells and a role for CD25 upregulation and IL-2 in enhancing these responses (8, 11, 20).