Response of the toad, Xenopus laevis, to circulating antigens. I. Cellular changes in the spleen

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Abstract
The localization, cytology and proliferation of lymphoid cells in the spleen of Xenopus laevis were investigated in normal toads and in toads stimulated with sheep erythrocytes (SRBC) or human gamma globulin (HGG). In toads given HGG in adjuvant, considerable increase in spleen weight was found both before and during serum antibody production; autoradiographic and histological examination showed that cell proliferation and formation of pyroninophilic cells had markedly increased in the splenic lymphoid tissue, particularly in the white pulp, but to a considerable extent also in the red pulp. Cell proliferation was also present, although less pronounced, after injections of HGG in saline. In toads given SRBC, most proliferation occurred in the white pulp, particularly towards the periphery of the follicles.Light and electron microscopy revealed that the lymphoid cells in the Xenopus spleen — in particular small lymphocytes and large pyroninophilic cells — are similar to those implicated in the mammalian immune response. However, cell clusters resembling mammalian germinal centres were absent. Furthermore, cells resembling the mammalian plasma cell were rare in both normal and immunized toads.