Comparative Immunohistochemical Detection of Major Immunocompetent Cells and Cell Proliferation in Peripheral Lymphoid Organs in Cattle and Yak (<i>Bos gruniensis</i>)

This study aimed to identify and analyse the distribution and functional activity of the main types of immunocompetent cells and to assess the level of cell proliferation in lymph nodes and spleen, as well as to identify species-specific differences in the immune system. The main results of the study detailed how T-lymphocytes, B-lymphocytes and macrophages organise and function in the lymph nodes and spleen, identifying specialised zones for cell-mediated and humoral immune response. A high concentration of T-lymphocytes was found in the paracortical (T-dependent) zone of lymph nodes, whereas B-lymphocytes were predominantly localised in lymphoid follicles (B-dependent zone), highlighting the specific organisation of immune defence. The spleen, which provides both humoral and cell-mediated immune response, showed the presence of periarteriolar lymphoid sheaths rich in T-lymphocytes (T-dependent zone) and lymphoid follicles with a high concentration of B-lymphocytes (B-dependent zone). In addition, the presence of macrophages, which play a key role in phagocytosis and antigen presentation, was confirmed in both organs. Immunohistochemical analysis revealed active proliferation of immunoblasts in response to immune challenges, indicating the dynamic nature of the immune response. These data emphasise the complexity and efficiency of the immune system of the studied species, providing their protection against infectious agents and maintaining their health, which is key for the development of new approaches to improve animal health and productivity.