Question

State the functions of primary and secondary lymphoid organs in humans

Answer 1

Hint: Lymph is a fluid connective tissue. It circulates in the body and finally enters the blood circulation. This is known as the Lymphatic system. It includes Lymph vessels, Lymph nodes, and Lymphatic organs.

Complete answer:
Blood continuously flows through the blood vessels. When it flows through the capillaries, some amount of blood oozes out. This is known as tissue fluid. This fluid contains some lymphocytes too. Lymphocytes are the immune cells and if they ooze out they will not be able to perform their function. To avoid this the tissue fluid enters the Lymphatic vessels and this fluid is not termed as Lymph. The Lymph circulates through the Lymphatic system and finally enters the blood. The immune cells either get stored in lymph nodes or lymphoid organs for further development.

These Lymphoid organs are of two types:
Primary and Secondary.
1) Primary lymphoid organs are the central organs. The main function is to generate the Lymphocytes (B and T cells). The primary lymphoid organs are also responsible for the maturation of these lymphocytes. The B lymphocytes form and mature in the bone marrow while T lymphocytes form in the bone marrow and mature in the thymus gland. Thus Thymus and the Bone marrow are primary lymphoid organs. These organs do not interact with Antigens at all.

2) Secondary lymphoid organs are those organs that are responsible for initiating the Adaptive immunity. Adaptive is Acquired immunity in which the Antigen triggers the formation of Antibodies (seen in Antibody-mediated Acquired immunity) or the pathogens directly trigger the immune cells (seen in Cell-mediated Acquired immunity). Thus the B cells and T cells after maturation in primary organs are stored in secondary organs and are ready for action. The interaction of immune cells with each other is also seen in these organs.
These organs include the spleen, lymph nodes, MALT, tonsils, Peyer's patches, etc.
(MALT is Mucosal Associated Lymphoid Tissue is seen in mucosa of Alimentary and Respiratory tract and contains immune cells).

Note: The lymphoid system collects the tissue fluid which has many fighting cells and thus helps in immunity. This system was explained by Olaus Rudbeck and Thomas Bartholin.
With age the primary lymphoid organ shrinks and loses its strength and the secondary becomes more active.