Model of human NK-cell development. (1) Bone marrow–derived CD34⁺CD45RA⁺ HPCs circulate in the blood and (2) extravasate across lymph node high endothelial venules to enter the parafollicular space. There, (3) pro-NK cells are activated to progress through distinct stages of maturation (far right) to create both CD56^bright and CD56^dim NK cells.³¹  Maturing CD56^dim NK cells return to the circulation via the efferent lymph (4),³²  whereas some CD56^bright NK cells remain within the secondary lymphoid tissue to interact with DCs (5).^21,23,33,34  Illustration by Debra T. Dartez.