The primary lymphoid organs are bone marrow and thymus where immature lymphocytes differentiate into antigen sensitive lymphocytes.

Bone marrow : Bone marrow is the main lymphoid organ where all blood cells including lymphocytes are formed. Maturation of B-lymphocyte occurs here.

Thymus : Thymus is the site for T-lymphocytes maturation. Thymus is situated near the heart.

The secondary lymphoid organs are spleen, lymph nodes, tonsils,Payer's patches of small intestine and appendix.

Spleen : It is a bean-shaped organ which is the largest single mass of lymphoid tissue in the body. In foetus the spleen produces all type of blood cells but in adult it only produces lymphocytes.

Lymph nodes : These are small solid structures found at intervals along the lymphatic system. They are composed of lymphoid tissue and act as filters for the lymph. Lymph nodes also produce lymphocytes and plasma cells.

Tonsils : Usually there are only 6 tonsils. They act as filters to protect the body from bacteria and aid in the formation of white blood cells.

Peter's patches : They are clusters of lymph nodules found in small intestine along the ileum. They produce lymphocytes.

Mucosal-Associated Lymphoid Tissue (MALT) : MALT are significant aggregation of lymphoid tissues which are seen in relation to the mucosa of the major tracts like respiratory, alimentary canal and urinogenital tracts. It constitute about 50% of the lymphoid tissue in human body. They don't serve as filters of lymph. They are centres of lymphocytes production. Phagocytic macrophages and dendritic cells are also present.