Lymphoid organs are those organs where origin and/or maturation and proliferation of lymphocytes occurs e.g., bone marrow, spleen, lymph nodes, thymus. Thymus is a primary lymphoid organ (located near heart beneath breast bone), where development and maturation of T-lymphocytes occurs. Spleen and lymph nodes are secondary lymphoid organs which provide the sites for interaction of lymphocytes with antigen, which then proliferate to become effector cells. Kidney is an excretory organ (present in pair) which is responsible for excretion of wastes, principally urea from blood.