The predominant gut-infiltrating proinflammatory T cell populations in IBD are Th1 cells, which express the transcription factor T-bet and the cytokine interferon (IFN-)γ, and Th17 cells, which express the transcription factor retinoid (RAR)-related orphan receptor (ROR)γt and the cytokine IL-17 [29,30,31,32,33,34]. Here, IL17A is linked to inflammatory bowel disease.