Histology of the lymphoid system
The lymphoid system is a crucial component of the immune system, encompassing the spleen, thymus, tonsils and lymph nodes. It plays a pivotal role in defending the body against infections by producing and circulating lymphocytes. Additionally, the lymphoid system helps maintain fluid balance by returning excess fluid from tissues back into the bloodstream. This intricate network of organs and vessels contributes to the body's ability to ward off pathogens and maintain overall health.
Lymph nodes are secondary lymphoid organs and play a central role in the body's immune defense. They consist of an outer capsule made of collagenous connective tissue, from which trabeculae or septa extend into the body of the node, which divide the organ into compartments. Three main areas can be distinguished inside the lymph node: the outer, dark cortex (B zone), the parafollicular zone (T zone) and the inner, lighter medulla.
Lymph enters the lymph node via afferent lymphatic vessels through the organ capsule and flows through a lymphatic vessel system called the sinus system. There the lymph is filtered and cleaned of antigens and leaves the lymph node again at the hilum through efferent lymph vessels.
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The tonsils are a collection of lymphoid tissue found in the mucosa of the pharynx which form an important part of our immune system and act as the first line of defense against ingested or inhaled pathogens. There are four primary groups of tonsils: pharyngeal, tubal, palatine and lingual, which are arranged into a ring around the naso- and oropharynx and are collectively known as the pharyngeal lymphoid ring (of Waldeyer).
The surfaces of palatine and lingual tonsils are interrupted by a variable number of tonsillar pits that open into deep invaginations known as tonsillar crypts, which can accumulate detritus and microorganisms.
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The thymus is an encapsulated primary lymphoid organ. Histologically, it is divided into subcapsular cortical, inner cortical and medullary regions within each lobule, created by the intervening connective tissue septae extending from the capsule.
The capsule is made up of inner and outer layers of collagen and reticular fibers, with lymphocytes found in between.
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The spleen is the largest lymphoid organ in the human body and plays an important role in immune defense and the filtration of old and damaged erythrocytes. It is divided into the red pulp and white pulp. Its outer surface is surrounded by a tough capsule from which trabeculae extend into the parenchyma of the spleen.
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