Age-associated changes in NK cell function. Example flow cytometry plots are shown for CD3⁻ lymphocytes from a 1-year-old (A) and a 22-year-old (B), cultured in medium alone (top) or stimulated with high concentrations of IL-12+ IL-18 (HCC, bottom) and assayed for degranulation (CD107a), CD25, and IFN-γ expression. (C-K) NK cells were assayed for degranulation (C-E), CD25 (F-H), or IFN-γ (I-K) expression after in vitro culture in medium alone (C,F,I) or with K562 target cells (D,G,J) or IL-12 + IL-18 (HCC; E,H,K). Horizontal bars represent median values, boxes extend from the 25th to the 75th percentile, and whiskers represent the 95th percentiles. Asterisks denote significant age-related trends for frequencies of NK cells expressing CD107a, CD25, or IFN-γ (**P < .01;*** P< .001, F-test).