The human immune system is an important system, which facilitates major defensive actions against the foreign particles and microorganisms. The tissue aggregates where leukocytes mature, differentiate, and proliferate are called the lymphoid organs. They are mainly composed of epithelial cells and stromal cells, arranged either into organs or accumulation of diffuse lymphoid tissues. Lymphoid organs are classified as primary and secondary lymphoid organs.
Primary Lymphoid Organs
Primary lymphoid organs include the thymus and bone marrow. They are the places where the B and T lymphocytes differentiate from stem cells; therefore, called as the sites of lymphopoiesis. These organs were first discovered in birds, in which the maturation of B lymphocyte takes place in the bursa of Fabricius. Humans do not possess this organ. In humans, B lymphocytes mature and differentiate from hematopoietic stem cells in the fetal liver during the embryonic life. After birth, the maturation and differentiation of B cells take place in the bone marrow. Progenitor cells of bone marrow differentiate into T lymphocytes, once they migrate to the thymus. Thus, the major function of the thymus is to direct T lymphocytes to differentiate between self and nonself antigens.
Secondary Lymphoid Organs
Secondary lymphoid organs include the lymphoid nodes, Peyer’s patches, spleen, tonsils and adenoids. They are the sites where the antigen- driven proliferation and differentiation, and lymphocyte respond to pathogens and foreign antigens take place. Infectious organisms are likely to be found in these organs.
What is the difference between Primary and Secondary Lymphoid Organs?
• Primary lymphoid organs develop before secondary organs during the ontogeny.
• Primary lymphoid organs are the thymus and bone marrow, whereas secondary lymphoid organs are the lymphoid nodes, Peyer’s patches, tonsils, adenoids and spleen.
• Primary lymphoid organs are the site of maturation for T and B cells, whereas secondary lymphoid organs are the sites of cell function for mature T and B cells.
• Differentiation of lymphocytes is taken place in primary lymphoid organs while the interaction of immune cells with each other and antigen processing are taken place in secondary lymphoid organs.
• Primary lymphoid organs serve as the microenvironment for antigen- independent differentiation of lymphocytes, whereas secondary lymphoid organs serve as the microenvironment for attracting antigen- specific lymphocytes, facilitating the lymphocyte differentiation and distributing the differentiated effector cells or their products to other parts of the body.
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