Vitamin D is an important nutrient controlling the health and development of our bones. Many patients with inflammatory bowel disease (IBD) are deficient in levels of vitamin D in their bodies. This is probably because vitamin D is lost from inflamed intestinal tissue into the stools. But while much attention has been given to studying the impact of vitamin D deficiency on the bone status of patients with IBD, our understanding of how vitamin D deficiency might affect the immune system in these patients is relatively poor.