Abstract. Eleven cases of alimentary lymphoma affecting the ileum were observed among 26 cases of swine lymphoma detected by meat inspection in Kochi, Japan. The ileal lymphomas were located in the Peyer's patches, along with early involvement of regional lymph nodes, and showed a characteristic pattern of follicular invasion leading to diffuse growth. Following the National Cancer Institute Working Formulation, 10 neoplasms were classified as diffuse, large, noncleaved cell lymphomas and one neoplasm was a diffuse, mixed, small to large cell lymphoma. Both types of lymphoma featured numerous intermingled "starry sky" histiocytes. The lymphoma cells tended to infiltrate into the muscular layer of the ileum in an "Indian file" pattern. Two cases also showed transserosal metastasis into the abdomen and leukemic change. The lymphoma cells showed membrane positivity for alkaline phosphatase and diffuse cytoplasmic staining for acid phosphatase and nonspecific esterase. Monoclonal intracytoplasmic immunoglobulins were demonstrated in nine neoplasms (IgM-X in seven, IgG-X in one, and IgG-K in one). In the areas of follicular invasion, an attenuated network of follicular dendritic cells was visualized via an antiserum against the p subunit of S-100 protein. Ultrastructurally, strands of dilatated rough endoplasmic reticulum and scattered or clustered dense bodies were noted. When compared with feline and human alimentary lymphoma, including Burkitt's lymphoma, the present neoplasms possessed distinctive features, such as originating in Peyer's patches, transserosal metastasis, and predominantly large B cell type with IgM-X type immunoglobulin expression, although some features were similar.

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