Light micrograph of a part of the wall of the human small intestine in Crohn's disease, showing a granuloma located between the muscle layers. Crohn's disease is a chronic inflammatory disease of unknown cause that mainly affects the small intestine, although the colon & anus may also be involved. Affected gut becomes thickened & pitted with fissured ulcers. Granuloma formation occurs throughout all layers of the gut wall. This oval granuloma contains macrophages and giant cells (dense purple stain) as well as smaller, darkly- stained lymphocytes. One consequence of long- standing Crohn's disease is fibrosis that may cause bowel obstruction.