Primary small intestinal lymphoma (PSIL) represents a heterogenous group of disorders with variable clinical and pathologic features and a characteristic age, socioeconomic, and geographic distribution. In developed countries, PSIL usually occurs as a localized ileal tumor, shows a bimodal age distribution, and most frequently presents with abdominal pain and obstructive symptoms. Histologicaily, most of these tumors are diffuse histiocytic, lymphocytic, or undifferentiated lymphomas. Other variants of PSIL, collectively referred to as immunoproliferative small intestinal disease, occur most often among young patients of poor socioeconomic status in Third World countries, mostly in the Middle East and Mediterranean area. They are characterized by involvement of long loops of the upper small intestine and commonly present with abdominal pain, diarrhea, malabsorption, and clubbing of the fingers. A subgroup of these patients shows a serological abnormality with the appearance of part of the alpha heavy chain of IgA in the serum. Histologically, the lesion appears as a dense diffuse lymphoplasmacytic infiltrate of the mucosa of the upper jejenum or duodenum. A form of malignant lymphoma of true histiocytic origin complicates long‐standing celiac disease. The contrasting clinical, epidemiological, histopathological, and immunological features of these variants of PSIL raise interesting questions about the pathogenesis of small bowel lymphoma.
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