Does the failure to acquire helminthic parasites predispose to Crohn's disease?

Two polarized patterns (Th1 and Th2) of cytokines regulate inflammatory res ponses. Each cytokine pattern inhibits production of the opposing pattern. Lymphocytes from inflamed intestine due to Crohn's disease secrete a Th1 pa ttern of cytokines. Crohn's disease is most prevalent in highly industriali zed countries with temperate climates. It occurs rarely in tropical third w orld countries with poor sanitation. We propose that exposure to an environ mental agent predisposes individuals to Crohn's disease. Parasitic worms (h elminths) are common in tropical climates and in populations subject to cro wding and poor sanitation. Children are most subject to helminthic coloniza tion. Many helminths live within or migrate through the human gut where the y interact with the mucosal immune system. The host mounts a mucosal respon se that includes Th2 cytokine production limiting helminthic colonization. Helminths and their eggs probably are the most potent stimulators of mucosa l Th2 responses. The Th2 response provoked by parasitic worms can modulate immune reactions to unrelated parasitic, bacterial, and viral infections. M any people in developed countries now live in increasingly hygienic environ ments, avoiding exposure to helminths. Perhaps failure to acquire these par asites and experience mucosal Th2 conditioning predisposes to Crohn's disea se, which is an overly active Th1 inflammation.