Secondary lymphoid tissue lymph nodes

In the specialized environment of secondary lymphoid tissues such as lymph nodes or spleen, dendritic cells provide the requirements for naive T-lymphocytes to become activated and to proliferate. The professional antigen-presenting cells present peptides in MHC II, express costimulatory molecules, and release cytokines into the immunological synapse, which is formed by the antigen-presenting cell and the naive T-lymphocyte. Thus, cells of innate immunity initiate and facilitate the activation of naive lymphocytes, and it is easily conceivable that their cytokines and adhesion molecules will instruct the naive T-lymphocyte during activation and differentiation to T-effector cells. 

The immune system functions to protect the organism against infection, foreign proteins, and neoplastic cells. It is organized into primary lymphoid tissue such as bone marrow and thymus and secondary lymphoid tissues such as spleen and lymph nodes. The cells of the immune system are leukocytes and a specific type produces immunoglobulins. 

Stimulation of the human immune system through the lymphoid tissue of the intestines can occur directly or indirectly. Such stimulation may involve the effect produced on the primary (bone marrow, thymus, and liver) and secondary organs (spleen, lymph nodes, tonsils, lymphoid tissue of bronchi, intestines, and skin) and... 

The lymphoid tissues include primary (or central) lymphoid organs (bone marrow, thymus and bursa of Fabricius in birds), and secondary (or peripheral) lymphoid organs (lymph nodes, spleen, tonsils, appendix, Peyer s patches). 

Saeki, H., Moore, A. M., Brown, M. J., and Hwang, S. T. (1999). Secondary lymphoid-tissue chemokine (SLC) and CC chemokine receptor 7 (CCR7) participate in the emigration pathway of mature dendritic cells from the skin to regional lymph nodes. J. Immunol. 162, 2472-2475. 

After maturation in the bone marrow, B lymphocytes home into secondary lymphoid tissues, such as lymph nodes, from which they are soon released into the circulation, or move through different areas while undergoing a final differentiation (3). The chemokine receptor CXCR5 may be involved in B-lymphocyte entry into the B-lymphocyte-rich follicles. CXCR5 is expressed on the surface of mature circulating B lymphocytes (3,35,60). Its ligand, CXCL13, is constitutively expressed within the follicles (2,3,61). 

Dendritic cells are found in tissues in their immature form. When they contact an antigen, the cell is activated, phagocytosis occurs, and a peptide-MHC complex is formed. Once activated, the newly mature dendritic cells travel to primary and secondary lymphoid tissues. Dendritic cells are found in afferent lymph nodes only, not efferent lymph nodes, indicating that most of the migrating dendritic cells die after their arrival in lymphoid tissues (67). 

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