Activated T cells can be identified immunohistochemically in the intestinal lamina propria in a number of gastrointestinal diseases including food sensitive enteropathy (coeliac disease) and intractable diarrhoea of infancy. Experimental studies have shown that T cell activation in human intestinal lamina propria in vitro produces an increase in crypt cell proliferation, villous atrophy, increased HLA-DR expression on enterocytes, increased intraepithelial lymphocyte numbers, and, phenotypically, macrophage activation. All of these features are seen in food sensitive enteropathy and it is proposed that lamina propria T cell activation to food antigens plays the primary role in the pathogenesis of these disorders by altering mucosal morphology and the rate of epithelial cell proliferation.