Lymphoid Organs

The lymphoid organs are complex tissues where lymphocytes make interactions with non-lymphoid cells, such as dendritic cells and macrophages, that are essential for lymphocyte development or to initiate the adaptive immune response. They are categorized into primary or central lymphoid organs, where the lymphocytes are created and secondary or peripheral lymphoid organs, where the adaptive immune system responses are started. The thymus is a primary lymphoid organ, the site where T cells develop. Secondary lymphoid organs viewed include the spleen, Peyer's patches and lymph nodes (e.g. peritoneal lymph nodes). The spleen is a fist-sized organ lying behind the stomach that contains red pulp the site of red blood cell destruction and lymphoid white pulp, which consists of lymphocytes responsible for initiating immune responses toward antigens in the blood. The Peyer's patches are highly organized structures that form the majority of the gut-associated lymphoid tissues (GALT). These patches are comprised of M cells which collect antigen from the epithelial surfaces of the gastrointestinal tract and initiate adaptive immune responses, with the lymphocytes found present in the resident follicles. The lymph nodes (e.g. peritoneal, mesenteric) are secondary lymphoid organs that are the sites of convergence of vessels that collect the extracellular fluid from the tissues and recirculate it into the blood. Lymph nodes act as filters in the lymphatic system and are capable of initiating adaptive immune responses to antigens found in the lymph, which was circulated through from tissues. In older aged mice, the Peyer's patches and lymph nodes are difficult to be seen because these tissues decrease in size with age. The spleen and lymph nodes are both encapsulated by connective tissue and by fibroblasts, while the Peyer's patches are not (1).

References:

1. Janeway, C.A., Travers P., Walport M., and Capra J.D. 1999. Immunobiology: The immune system in health and disease. Garland Publishing, 4th ed., New York, USA, pages 1-40, 2.5-2.22, 3.1-3.12

2. DeFranco, A.I. 1987. Molecular aspects of B-lymphocyte activation. Annu. Rev. Cell Biol. 3:143-178

3. MacLennan, I.C.M. 1994. Germinal centers. Annu. Rev. Immunol. 3:117-139

4. Ravetch, J.V. and Kinet, J. 1993. Fc receptors. Annu. Rev. Immunol. 9:457-492