Question

Explain the role of Primary and Secondary Lymphoid organs with the help of suitable examples.

Answer 1

• Lymphoid organs: These are the organs where the origin and/or maturation and proliferation of lymphocytes occur.
• The primary lymphoid organs are bone marrow and thymus where immature lymphocytes differentiate into antigen-sensitive lymphocytes.
• After maturation, the lymphocytes migrate to secondary lymphoid organs like the spleen, lymph nodes, tonsils, Peyer’s patches of small intestine and appendix.
• The secondary lymphoid organs provide the sites for the interaction of lymphocytes with the antigen, which then proliferate to become effector cells.
• The bone marrow is the main lymphoid organ where all blood cells including lymphocytes are produced.
• The thymus is a lobed organ located near the heart and beneath the breastbone.
• Both bone marrow and thymus provide micro-environments for the development and maturation of T-lymphocytes.
• The spleen is a large bean-shaped organ. It mainly contains lymphocytes and phagocytes.
• It acts as a filter of the blood by trapping blood-borne microorganisms. Spleen also has a large reservoir of erythrocytes.
• The lymph nodes are small solid structures located at different points along the lymphatic system. Lymph nodes serve to trap the micro-organisms or other antigens, which happen to get into the lymph and tissue fluid. Antigens trapped in the lymph nodes are responsible for the activation of lymphocytes present there and cause the immune response.
  There is lymphoid tissue also located within the lining of the major tracts (respiratory, digestive and urogenital tracts) called mucosa-associated lymphoid tissue (MALT). It constitutes about 50 per cent of the lymphoid tissue in the human body.