Antibody reagents were made specific for each of the two forms of human SC, FSC and BSC, which is an integral part of the sIgA molecule. With the fluorescent antibody method the cytological and histological localization of FSC, BSC, and α-chains has been studied in various human mucous membranes. SC was present in columnar epithelial cells of the intestines and in the cells of serous acini of bronchial and salivary glands. In contrast, SC was not found in intestinal goblet cells or cells of mucous acini of bronchial and salivary glands. In the columnar epithelial cells of the small and large bowel, FSC was present most prominently in the Golgi zone, and much less prominently in the apical cytoplasm. On the other hand, BSC and α-chains were located only in the apical cytoplasm in an overlapping manner. The results favor a model in which sIgA is assembled inside epithelial cells from SC, which was synthesized in the same cell, and IgA, which entered the epithelial cell after synthesis in and secretion by a plasma cell.

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