8.2.7 IMMUNE SYSTEM IN THE BODY

8.2.7 Immune System in the Body 

  • The human immune system consists of lymphoid organs, tissues, cells and soluble molecules like antibodies. 
  • As you have read, immune system is unique in the sense that it recognises foreign antigens, responds to these and remembers them. 
  • The immune system also plays an important role in allergic reactions, auto-immune diseases and organ transplantation. 
Lymphoid organs: 

  • These are the organs where origin and/or maturation and proliferation of lymphocytes occur. 
The primary lymphoid organs 

  • The primary lymphoid organs are bone marrow and thymus where immature lymphocytes differentiate into antigen-sensitive lymphocytes. 
  • The location of various lymphoid organs in the human body is shown in Figure below-
                                                         NCERT Class XII Biology Chapter 8 : Human Health And Disease ...
  • 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. 
  • The thymus is quite large at the time of birth but keeps reducing in size with age and by the time puberty is attained it reduces to a very small size. 
  • Both bone-marrow and thymus provide micro-environments for the development and maturation of T-lymphocytes. 

Secondary lymphoid organs:
  • The secondary lymphoid organs provide the sites for interaction of lymphocytes with the antigen, which then proliferate to become effector cells. 
  • After maturation the lymphocytes migrate to secondary lymphoid organs like spleen, lymph nodes, tonsils, Peyer’s patches of small intestine and appendix. 
The spleen 

      • 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
      • 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.
      Mucosa Associated Lymphoid Tissue (MALT).
      • 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 human body.

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