(a) Lymphoid organs 

(i) Bone marrow 

• It is the main lymphoid organ, whose all types of blood cells are formed.
• It provides the micro environment for the development and maturation of B-lymphocytes. 

(ii) Thymus 

• It is located beneath the chest bone near the heart.
• This gland keeps reducing in size with age. 
• It provides the micro environment for the development and maturation of T-lymphocytes. 

(iii) Spleen 

• It contains mainly lymphocytes and phagocytes.
• It acts as a filter of blood by trapping the blood-borne pathogens. 

(iv) Lymph nodes

• Lymph nodes are small, solid structures, found at different points along the lymphatic system.
• They act as filters and trap the microbes that have entered the lymph. 
• Antigens trapped in them activate the lymphocytes present in the lymph nodes and produce an immune response. 

(b) Bone marrow and thymus are the primary lymphoid organs, where the immature lymphocytes undergo differentiation into antigen-sensitive lymphocytes.

Spleen and lymph nodes are secondary lymphoid organs, where the lymphocytes interact with the antigen and proliferate to form a clone (effector and memory cells).