The route of migration of lymphocytes from blood to spleen and mesenteric lymph nodes in the pig.

Porcine peripheral blood lymphocytes were labelled with tritiated uridine in vitro and transfused into normal young pigs. Serial biopsies were taken from the spleen and mesenteric lymph nodes, and the localization of labelled lymphocytes determined on autoradiographs. Four to five hours after cell infusion samples of Peyer's patches, tonsils and small intestine were taken and the labelled lymphocytes classified by their localization in different organ compartments. In spleens most labelled cells were found in the red pulp and about 30% in the marginal zone. Within 4 to 5 h there was a constant increase in labelled lymphocytes in the compartment of the white pulp with more cells localized in the periarteriolar lymphatic sheath than in the corona. In lymph nodes about two thirds of all labelled cells were seen in the wall of high endothelial venules 15 min after cell infusion. Labelled cells then decreased in the vessel wall, but, at the same time, increased in the neighbourhood of these vessels and other parts of the paracortex. After 5 h about 10% of labelled lymphocytes had already migrated into the corona. In tonsils and Peyer's patches a comparable distribution of labelled cells was found in the organ compartments. The kinetics and the route of migration in the spleen and lymph nodes are therefore comparable to data for other species, despite the peculiar structure of pig lymph nodes and the paucity of lymphocytes in efferent lymphatics in pigs.