The lymphoid organs are part of the immune system. They are classified into primary and secondary organs. Primary lymphoid organs are where lymphocytes (B cells and T cells) are produced and mature. These are the Bone marrow (site of production of all blood cells, including lymphocytes, and B cell maturation) and the Thymus (site of T cell maturation). Secondary lymphoid organs are where lymphocytes are activated and proliferate in response to antigens. These include the Spleen, lymph nodes, tonsils, adenoids, appendix, and Peyer's patches in the small intestine.