Small intestinal granulomas form in many disorders (Table 6.15). The nature of the granulomas varies significantly from small macrophage collections that may or may not contain foreign material to classic granulomas. Small crypt-associated mucin granulomas complicate mucosal injury. Histiocytes may lie under the free surface or occupy the lamina propria and may
contain microorganisms. Vessel-associated histiocytic collections complicate infections, especially CMV (Fig. 6.119). Histiocytic cells also surround the air spaces, characteristic of pneumatosis intestinalis (Fig. 6.208), or they form small mucosal collections in transplant patients without evidence of infection. They also aggregate in the lamina propria (Fig. 6.209) or submucosa in patients with xanthomas. The etiology of granulomas or histiocytic collections differs depending on whether they are compact or loose, are diffuse or localized, contain giant cells or not, and associate with areas of necrosis or not. Peritoneal rosetting microgranulomas may also be encountered in incarcerated small intestinal loops. The rosetted cells are CD68+, distinguishing them from areas of reactive mesothelial hyperplasia (578).