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The lymphoid tissue consists of mesh-like areas of connective tissues within the body containing white blood cells, most commonly lymphocytes. This tissue and lymphatic vessels, which transport clear body fluid called lymph to the heart, comprise the lymphatic system. Primarily involved with immune function, the components of the lymphoid tissue include the lymph nodes, the tonsils and adenoids, the spleen, and the mucosa-associated lymphoid tissues (MALT). In addition, the thymus gland and the bone marrow play a role in lymphoid function in the body. This tissue operates to defend the body from infections, foreign materials, and spreading cancer cells.
Three broad divisions of lymphoid tissues exist, which are organized based upon the stage of lymphocyte development that occurs within each one. The primary lymphoid organs, body sites involved with the generation of lymphocytes from progenitor stem cells, make functional white blood cells that are ready to respond but not targeted to respond to a particular foreign material or cell. Secondary lymphoid organs, such as the lymph nodes, keep these "naïve" lymphocytes, and, when exposed to an invading cell, activate the lymphocytes for action against that threat. Once the body activates a lymphocyte to respond to a given threat, other lymphocytes are recruited and similarly activated so that the body can mount an immune response. Finally, the tertiary lymphoid tissue imports activated lymphocytes from the blood and lymph in cases of active body inflammation.
Excess fluid in an organ drains as lymph through lymphatic vessels to a lymph node. Lymph nodes act as filters to screen harmful materials and cells from the lymph before it eventually reenters the blood stream. The lymph nodes occur most frequently in the chest cavity, the armpit, the groin, and the neck. Composed of two layers, the lymph node percolates fluid through its outermost layer, the cortex, in which concentrated pockets of lymphocytes, called follicles, stand ready to turn on if they encounter a foreign material.
The mucosa-associated lymphoid tissue (MALT) is a widespread network of tiny clumps of lymphoid tissue scattered throughout various body tissues, including the skin, lungs, eye, and digestive tract. Mucosal surfaces of the body, including the moist linings in the mouth, nose, and throat, are often the sites of entry of bacteria and viruses. The MALT contains a variety of white blood cell types capable of mounting a first line of defense against infectious agents. Lymphoma, cancer of lymphoid tissue, can form in any site of this tissue, including areas of MALT.