Mouse: Levels of Th17 cells and IL-17+ Tregs increased and Treg levels decreased after NEC induction compared to controls. Frequency of circulating CCR9+ IL-17+ Tregs increased after NEC inductions. Increased level of circulating CCR9+ IL-17+ Tregs correlated negatively to severity of intestinal damage.Human: Monocytes from NEC infants had higher TLR4 expression. CD4+ T cells of NEC patients had higher RORC and lower FOXP3 transcription than controls, and expression of IL-17 was increased in patients. Here, CCR9 is linked to necrotizing enterocolitis.