Electronmicroscopic study of gamma irradiated mouse spleen during primary immune response

An electronmicroscopic study of the mouse spleen immunocompetent cells during the productive phase of the primary immune response after sublethal gamma ray irradiation is carried out. For this purpose the animals were immunized with sheep red blood cells 24 hours after irradiation and sacrified on the 5th day after immunization. The number of small lymphocytes is reduced in all zones of the spleen. Only in the periarteriolar area the lymphoid sheaths are well outlines and the ultrastructure of the cells preserved. Three types of reticulohistocytic elements, according to their radiosensitivity are observed. The most radioresistant cells are the fixed ''dark'' reticular cells which do not complete phagocytosis. The ultrastructure of their nucleus and cytoplasm is not damaged. The macrophages are also quite resistant. The ''light'' reticular cells are the most radiosensitive. The chromatine of their nuclei is dispersed. The mitochondria are imbibed, with a reduced number of cristae. The cytoplasm contains many electron light vesicles, different in size. The changes in the processes of the dendridic cells in the spleen lymph follicles are of particular interest. Compared with the control animals the processes of dendritic reticular cells are markedly reduced. The postirradiation ultrastructural changes of the spleen cells indicate that parallel with the basic factor (the death of a considerable part of the small lymphocytes, precursors of the antibody-synthetizing cells) the reduced antibody-formation is due also to the limited capacity for ''traping'' the antigen on the processes of the dendritic follicular cells and to the reduced capacity of the reticulo-histocytic cells for antigen phagocytosis. The later is determined both by the damage of a considerable part of the phagocytes (radiosensitive ''light'' reticulo-histocytic cells) and by the blocking of the functionally undamaged phagocytes from ingested debris. (K.M.)