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Secondary lymphoid organs (SLOs) include lymph nodes (LNs), spleen, Peyer’s patches (PPs) and mucosal tissues- the nasal associated lymphoid tissue (NALT), adenoids, and tonsils. Less discretely anatomically defined cellular accumulations include the bronchus associated lymphoid tissue (BALT), cryptopatches, and isolated lymphoid follicles (ILFs). All SLOs serve to generate immune responses and tolerance. SLO development depends on the precisely regulated expression of cooperating lymphoid chemokines and cytokines LTα, LTβ, RANKL, TNF, IL-7, and perhaps IL-17. The relative importance of these factors varies between the individual lymphoid organs. Participating in the process are lymphoid tissue initiator (ltin), lymphoid tissue inducer (ltind), and lymphoid tissue organizer (lto) cells. These cells, and others that produce the crucial cytokines, maintain SLOs in the adult. Similar signals regulate the transition from inflammation to ectopic or tertiary lymphoid tissues (TLOs). The mammalian immune system, a cooperative endeavor between the innate and acquired arms, provides an optimal environment for defense against the invasion of pathogens at any site in the body. The sites of organized lymphoid cell accumulations are termed primary and secondary lymphoid organs (SLOs)3. Diverse populations of functionally mature, but naïve, lymphocytes are generated in the absence of foreign antigens in the primary lymphoid organs (thymus, fetal liver, bone marrow). These cells seed the SLOs to optimally respond to foreign invaders. Here, we review the structure, function, development and maintenance of SLOs where T and B cells encounter antigen to generate effector cells or tolerance. SLOs include the spleen, lymph nodes (LNs), Peyer’s patches (PPs), tonsils and adenoids, and, in the mouse, rat, and rabbit, the nasal associated lymphoid tissue (NALT). During antigenic challenge, additional lymphoid tissues are apparent in the lung (bronchus associated lymphoid tissues (BALT)) and intestine (isolated lymphoid follicles (ILFs)). The blood vessels of SLOs allow antigen access. LNs, which are also served by lymphatic vessels, are effective in mounting responses to antigens that are present in tissues. These antigens originate from foreign invaders that are transported by antigen presenting cells or are derived from self-antigens. The capacity to discriminate between dangerous foreign antigens and benign self-antigens relies on the antigen presenting cells and their state of activation and the recognition capacity of the naïve T and B cells. DCs constitutively sample self-antigens and migrate to draining LNs even in the steady state. Because most self-antigen bearing DCs in LNs are immature, they do not effectively activate naïve cells. They regulate self-reactive T cells by inducing anergy, clonal deletion, and/or expanding regulatory T cells. Tertiary lymphoid organs (TLOs), or more accurately, tertiary lymphoid tissues, are accumulations of lymphoid cells that arise in the adult. These ectopic lymphoid accumulations respond to environmental stimuli with chronic inflammation during microbial infection, graft rejection, or autoimmune disease. They are classified as lymphoid tissues because they resemble SLOs with regard to cellular composition and compartmentalization, chemokines, vasculature and function #LymphaticSystem #SecondaryLymphoidOrgans #lymphNodes #spleen #nasalAssociatedLymphoidTissueNALT #adenoids #andTonsils #PeyersPatchesPPsAndMucosalTissues
