RAG2 and colitis: Transfer of Tbx21–/– naïve CD4+ T cells into Rag2–/– mice gives rise to a higher proportion of IL‐17A‐producing CD4+ T cells and more severe colitis compared to mice receiving WT naïve CD4+ T cells [15, 26], suggesting that T‐bet restrains pathology in IL‐17‐driven colitis [26, 27, 28, 29].