CBSE Class 12-science Answered
The human immune system consists of lymphoid organs, tissue cells and antibodies.
It plays a key role in allergic reactions, autoimmune diseases and organ transplantation.
Lymphoid Organs:
Lymphoid organs are the organs where origin, maturation and proliferation of lymphocytes
take place.
Primary Lymphoid Organs - Bone marrow and thymus are the primary lymphoid organs.
Bone marrow is the main lymphoid organ where all blood cells including lymphocytes are produced.
Maturation of B-lymphocytes occurs.
Thymus is the site for the development and maturation of T-lymphocytes which is situated near the heart beneath the breastbone.
Secondary Lymphoid Organs - Spleen, lymph nodes, tonsils, Peyer's patches of small intestine and appendix are the secondary lymphoid organs.
Spleen contains lymphocytes and phagocytes and acts as a filter of the blood as it traps the blood-borne microorganisms.
Lymph nodes trap the antigens that enter the lymph and tissue fluid which activates the lymphocytes causing immune response and enlargement of lymph glands.
Tonsils also act as filters to protect the body from bacteria and assist in the formation of WBC.
Another secondary lymphoid organ is the mucosal-associated lymphoid tissues (MALT) which are located within the lining of digestive, respiratory and urinogenital tracts and constitute 50% of the lymphoid tissue.