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The spleen and the thymus are also lymphoid organs of the immune system. The tonsils are lymphoid organs that are also associated with the digestive system. Lymphoid tissues contain lymphocytes, and also contain other types of cells for support.
The mature lymphocytes from primary lymphoid organs migrate to secondary lymphoid regions through blood circulation to give protection from invading foreign agents. The secondary lymphoid organs are the regions, where the immune competent lymphocytes interact with foreign agents and execute immune response. Production of antibodies from activated B cells, activation of cytotoxic T cells in response to antigens takes place in the secondary lymphoid organs.