Human Immune system:  Components of immune system:-

• Lymphoid organs
• Immune cells
• Antibodies
• Lymphoid tissues

(i) Primary lymphoid organs

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

(a) Bone marrow

• Here, all blood cells including lymphocytes originate. It also provides the micro-environment for the development and maturation of B-lymphocytes.

(b) Thymus  

• Thymus is a lobed organ located near the heart. It provide micro-environment for maturation and development of T-lymphocytes.
• It is quite large at the time of birth but reduces with age.

(ii) Secondary lymphoid organs

The organs where lymphocytes interact with the antigen and proliferate to become effector cells.

 (a) Spleen

• It is a large bean-shaped organ.
• It acts as a filter of the body 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.