Lymphoid System: The primary and secondary organs

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The stem cells in the bone marrow differentiate into immune compliment lymphocytes in the primary or central lymphoid organs and then later move into the secondary lymphoid organs. This organization is important for complete interaction between T and B cells with macrophages in the induction of an effective immune response.

The primary lymphoid organs include the thymus, bursa Fabricius, and the bone marrow. In mammals, the bone marrow serves as a primary and secondary lymphoid organ. While the secondary organs include the encapsulated organs of the lymph, which consist of the spleen, bone marrow, and the lymph nodes; and the non-capsulated organs of the lymph which consist of the MALT and the NALT

Non-capsulated organs include most lymphoid tissues in the body which are either diffuse aggregates of lymphoid tissues containing germinal centers lining the walls of lachrymal glands, salivary glands, respiratory tract, gastrointestinal tract, and the urogenital tract. They are also found in the inner ear and the lactating breast.


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MALT is found at the initial point of entry of foreign pathogens and therefore provides local protection in the subepithelial mucosal areas of the body which have contact with the external environment. The NALT of the other hand comprises three pairs of tonsils that destroy foreign antigens at the upper entrance of the respiratory and digestive system (Waldeyer’s ring)

The Female reproductive system is also part of the common MALT Capable of mounting an effective immune response against infectious agents and occasionally sperm cells.