| Secondary lymphoid tissues
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| The secondary lymphoid tissues comprise lymph nodes, spleen, and mucosa-associated lymphoid tissues (MALT)
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| These tissues are functionally organized to enable the interaction of lymphocytes with other cells and the antigen, and it is at these sites that immune reactions actually develop. Common to them all is a degree of compartmentalization, with specific areas for T cells and B cells and areas of overlap where they interact.
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| Within the lymph node, the T-cell area is the paracortex and the B-cell area the follicular areas of the medulla. Here follicular structures of two types can be found: the unstimulated primary follicle; and stimulated secondary follicles, characterized by the presence of germinal centers. The spleen contains nonlymphoid tissue (the red pulp) as well as lymphoid areas - termed the white pulp. The white pulp surrounds the splenic arterioles and is called the periarteriolar lymphoid sheath. Within the white pulp, follicular B-cell areas are evident, and the T-cell areas lie between them in the interfollicular space. MALT comprises the lymphoid elements found adjacent to the mucosal surfaces lining the internal body surfaces; they are found at the entrance to the respiratory tract and gut, and include the tonsils and adenoids. Further down the digestive tract, unencapsulated aggregates of lymphoid cells referred to as Peyer's patches are found, overlain by specialized areas of epithelium for sampling the antigenic environment. Solitary lymphocytes are also found throughout the mucosal epithelium.
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| Traffic between the secondary lymphoid tissues is via the lymphatics and blood vessels
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| The site of antigen localization in the lymphoid tissues after antigens gain access to the body depends upon the route of exposure. Entry via the circulation leads to localization in marginal zone lymphatics of the spleen. Entry via the gut and dome epithelia leads to localization in the MALT. With entry
via the skin, the antigen may pass directly to the local lymph node via the lymphatics, or it can be taken up by the epidermal Langerhans cells, which then pass via lymphatics to the interdigitating cell-rich area of the paracortex of the local lymph node. Entry via other sites/tissues leads to the local lymph node.
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