In humans, polymorphisms in IL-10 and IL-10 receptor are associated with IBD, such as Crohn’s disease (CD) and ulcerative Colitis (UC).3 Similarly, rare loss-of-function mutations in IL-10 or IL-10 receptor cause severe, very early-onset IBD in infants and children.3 However, despite this essential role of IL-10 for the prevention of IBD, the role and possible deficits of IL-10 production by blood-derived effector memory CD4+ T cells in IBD have not been studied in detail. Here, CD4 is linked to ulcerative colitis.