i. Primary lymphoid organs 

• The organs where lymphocytes originate and mature to become antigensensitive, e.g., bone marrow and thymus, are called primary lymphoid organs. 

a. Bone marrow 

• It is the primary lymphoid organ where all blood cells including lymphocytes originate. 

• Bone marrow provides the micro-environment for the development and maturation of B-lymphocytes. 

b. Thymus 

• Thymus is a lobed organ located near the heart and beneath the breastbone. 

• It is quite large at the time of birth but reduces with age. 

• It provides the micro-environment for the development and maturation of T-lymphocytes. 

ii. Secondary lymphoid organs 

• The organs where lymphocytes interact with the antigen and proliferate to become effector cells, e.g., spleen, lymph nodes, tonsils, Peyer’s patches of small intestine and appendix are called secondary lymphoid organs. 

a. Spleen 

• It is a large bean-shaped organ and contains lymphocytes and phagocytes. 

• It acts as a filter of the blood by trapping blood-borne microorganisms. 

• It has a large reservoir of erythrocytes. 

b. Lymph nodes 

• These are small solid structures present at different points along the lymphatic system. 

• They trap the microorganisms or other antigens that enter the lymph and tissue fluid. 

• Antigens trapped in the lymph nodes activate the lymphocytes and produce an immune response. 

c. Mucosal associated lymphoid tissue (MALT) 

• It is formed of masses of lymphoid tissue lining the mucosa of respiratory, digestive and urinogenital tracts. 

• 50 per cent of the lymphoid tissue in human body is formed by MALT.