Cross-sections of the parasitic worm Trichuris in the lumen of the mouse gut (left). Trichuris lives partially embedded in host intestinal epithelial cells. Protective T helper cells secrete molecules called cytokines that induce cells in the gut to produce mucus (right, intestinal goblet cell, stained blue). This mucus production is typical of a Th2 response. In the case of the lung, this same type of response induces mucus production that contributes to breathing difficulties typically suffered during an asthma attack. The absence of this mucus response in mice lacking Notch is consistent with a role for this pathway in controlling Th2 inflammation.