In coeliac disease there is an abnormality of the intestinal mucosa which is caused by ingesting gluten. The intestinal lesion affects predominantly the proximal small intestine and the ileum is either normal or less severely involved than the jejunum. In some cases adaptive changes occur in the ileum, producing enhanced absorption in that region when there is malabsorption in the jejunum. The characteristic absorptive abnormality in coeliac disease is therefore <i>jejunal malabsorption </i>and <i>Heal hyperabsorption. </i>When such a situation develops it is possible that an individual with a flat jejunal mucosa may develop no symptoms of the disease, since the adaptive changes in the ileum compensate for the jejunal lesion. This may explain why in Western society there are probably more cases of coeliac disease undiagnosed in the community than have been treated by their doctors. The basic lesion in coeliac disease appears to be genetically determined and it is likely to be a failure to clear antigen which normally enters the lamina propria of the gut resulting in the formation of immune complexes with complement fixation at gut level.