Massive plasma cell infiltration of the digestive tract. Secretory component as the rate-limiting factor of immunoglobulin secretion in external fluids.

A 29-yr-old Tunisian man had a clinical immunoproliferative small intestinal disease, different from alpha-chain disease. Serum contained 52.5 mg/ml of polymeric immunoglobulin A (IgA). Immunohistochemistry revealed a massive diffuse polyclonal IgA (99%)-plasma cell infiltration in the small bowel mucosa, with a smaller increase of IgA-producing cells in gastric and colonic mucosae. Secretory IgA levels were normal in jejunal and bronchoalveolar secretions. However, both fluids contained polymeric IgA devoid of secretory component, and free secretory component was absent. This suggests that secretory component was the limiting factor in transport of IgA in the secretions. A relative deficiency in secretory component, as compared with the huge supply of polymeric IgA, may have limited the secretory component-mediated active transport of IgA into secretions. This resulted in the appearance of high levels of polymeric IgA, unlinked to secretory component, both in serum and in the jejunal and bronchoalveolar fluids.