Baikal seal - endemic and the only aquatic mammal of Lake Baikal . Since the lake is included in the List of sites of the world natural heritage of UNESCO, it seems relevant to study its inhabitants, in particular such a unique animal as the Baikal seal, in order to preserve the biological diversity of the lake. In postnatal ontogenesis, the mesenteric lymph nodes of the Baikal seal undergo the following age-related changes in structural organization: an increase in the relative area of the connective tissue component ; Reduction of the proportion of cortical substance ; decrease in the relative area of lymphoid nodules. There is a trend towards an age-related decrease in the cortical-brain index. In the mesenteric lymph nodes of the Baikal seal in the process of postnatal ontogeny, there is a decrease in the number of medium lymphocytes and reticular cells, an increase in the number of plasma cells. reduction in the number of large lymphocytes, an increase in the content of small lymphocytes in the cortical substance ; reduction in the number of destructively altered and destroyed cells in the centers of reproduction of lymphoid nodules ; Reduction in the number of small lymphocytes in the brain substance .