Many studies have provided evidence that the immune system is a key regulatory system of intestinal function. The interaction of immune cells with the gut epithelium plays an important role in host defence, acting to eliminate pathogens, antigens and other noxious material from the lumen of the gastrointestinal tract. During inflammatory conditions of the gut, the mucosa becomes packed with immune cells in close proximity to the enterocytes. Mediators released from these cells have profound effects on epithelial functions. The two main functions of the intestinal epithelium are to transport nutrients, ions and water, and to act as a barrier to prevent unimpeded uptake of antigenic material and microbes from the lumen. Both these functions are altered by immune reactions in response to various stimuli. Topics discussed include mast cells and epithelial function; mast cell-nerve interaction; mast cell activation; neutrophils, eosinophils and macrophages; T cells; and prostaglandins and immune cell activation.