Enlarged mesenteric lymph nodes

This photo shows the serosal (outside) surface of the ileum and the adjacent mesenteric lymph nodes (arrow). Lymph nodes function to trap bacteria that may penetrate the intestinal wall. In response to M. paratuberculosis infection mesenteric lymph nodes often become enlarged. The tissues in this picture came from an experimentally M. paratuberculosis-infected heifer (#13). The pattern of diagnostic tests results for this animal are shown in another photograph. Moderate enlargement, without caseous necrosis, of the lymph nodes is evident. From the serosal surface, the ileum appears normal. The mucosal (internal) surface is shown in another photograph.