Light micrograph of a section through a jejunum affected by celiac disease. The jejunum is the region after the duodenum at the beginning of the small intestine. The surface (top) is abnormally flat due to the loss (atrophy) of villi, the small finger-like projections that increase the intestine's surface area. Celiac disease is a genetic disorder in which the intestinal lining reacts to gluten, a protein found in wheat and other cereals. This causes malabsorption of nutrients from food and leads to mineral and vitamin deficiencies.