The terminal ileum is affected in patients with lymphocytic or collagenous colitis.

Lymphocytic colitis (LC) and collagenous colitis (CC) are diseases characterized by the presence of marked intraepithelial lymphocytosis. Both of these disorders affect primarily the colon. However, involvement of the distal small intestine has not been systematically studied.

Prevalence and significance of inflammatory bowel disease-like morphologic features in collagenous and lymphocytic colitis.

Collagenous colitis (CC) and lymphocytic colitis (LC) are clinical syndromes characterized by the presence of chronic watery diarrhea, few or no endoscopic abnormalities and biopsies that typically show normal crypt architecture, increased mononuclear inflammation in the lamina propria, absence of neutrophils, and increased intraepithelial lymphocytes. Patients with CC also have a thickened subepithelial collagen layer. We have noted, anecdotally, that biopsy specimens from some patients with CC or LC contain certain histologic features, such as Paneth cell metaplasia (PM), that are normally seen in inflammatory bowel disease (IBD), or other types of healed colitis, and thus may cause diagnostic difficulty.