Plasticity and heterogeneity of lymphoid organs. What are the criteria to call a lymphoid organ primary, secondary or tertiary?

Abstract

Lymphoid organs are generally classified in a hierarchy with primary lymphoid organs such as the thymus and bone marrow for the production of receptor specific T and B lymphocytes, respectively, independent of antigens. In secondary lymphoid organs such as lymph nodes, spleen, and tonsils, the lymphocytes are expanded due to antigen exposure, producing memory T cells and effector B cells, resulting in plasma cells. Tertiary lymphoid tissues are often defined as aggregations of lymphoid cells in autoimmune diseases. It will be outlined that all these organs have a high plasticity and also the thymic medulla is included in the route of migrating mature T cells and the bone marrow, not only in the traffic of CD4+ but also of CD8+ lymphocytes. The mucosa-associated lymphoid organs depend to a much larger extent on microbial antigen and are much more diverse than often described. The role of structural elements as well as blood and lymphatic vessels as an entry and exit site of lymphocytes will be outlined. Using a precise terminology, taking account of the plasticity of these organs at different ages and considering species differences will reduce misunderstandings among immunologists.