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The primary lymphoid organs are bone marrow and thymus, where immature lymphocytes differentiate into antigen sensitive lymphocytes. They migrate to secondary lymphoid organ like spleen, lymph nodes, tonsils, Peyer's patches of small intestine and appendix. All blood cells including lymphocytes are produced in bone marrow which is main lymphoid organ. Spleen is large reservoir of erythrocytes. It is large bean shaped organ mainly containing lymphocyte and phagocyte. It acts as filter of blood by trapping blood borne micro organism. Lymph nodes are small solid structures located at different points along lymphatic system. Antigens trapped in lymph node are responsible for activation of lymphocytes present there and cause the immune response. There is lymphoid issue located within lining of major tracts like respiratory, digestive and urinogenital tracts called mucous associated lymphoid tissue (MALT).