The Natural Killer [NK] Cell plays a critical role in regulating the innate and adaptive immune response to pathogens, injury and stress. Although originally portrayed as primarily a lytic cell, the NK cell has emerged as a cell capable of helper function, expansion, contraction, and accelerated memory responses - features similar to other adaptive immune cells. It is a professional accelerator of immunity, mediating dendritic cell maturation and its precursors critical for the origin and development of secondary lymph node structures. NK cells also play an essential role in mammalian placenta.