The lymph nodes are major organs that perform many functions within the lymphatic system. Primarily, they remove debris and pathogens from lymph or tissue fluid. Some people describe the lymph nodes as lymph filters because of this primary function. The organs contain many cells that can internalize and kill pathogens that pass through. Additionally, the lymph nodes act as the site for adaptive immune responses involving special types of white blood cells.
The human body contains around 700 lymph nodes throughout the neck, groin, general intestinal tract, and other areas. The nodes have a bean or kidney-like shape and are generally one or two centimeters in length. A fibrous capsule surrounds each node. The lymph nodes themselves have two sections: the outer cortex and the inner medulla. The cortex surrounds the medulla except for the areas where vein-like lymphatic vessels connect. A supporting mesh called a reticulin sits in and supports the lymph nodes.
Many facets and processes make up the immune system. The lymph nodes play major roles in the slow, but significantly effective, adaptive immune response. Many different cells contribute to this process, but lymphocytes are the primary controllers. A special type of white blood cells, lymphocytes include B cells and T cells. Plasma cells and natural killer cells are more advanced lymphocytes.
Interstitial fluid exists in the spaces around cells. It helps carry oxygen and nutrients to cells and remove waste from them. The interstitial fluid becomes lymph when it enters the lymphatic system through the lymph capillaries. Because lymph originates from the interstitial fluid, its composition is ever-changing. Blood and the surrounding cells constantly exchange substances with interstitial fluid, and lymph transports many of these substances. For example, it returns proteins and excess interstitial fluid to the bloodstream while simultaneously collecting bacteria for delivery to the lymph nodes.
Dense irregular connective tissue and plain collagenous fibers compose the lymph node capsule. Connective tissues called trabeculae slightly divide areas in the nodes into compartments. Eventually, the trabeculae meld into the tissue that connects the node with the lymphatic vessels. This creates a framework for the node, as well as a careful arrangement of connective tissue that forms the subcapsular sinus, also known as the lymph sinus or marginal sinus.
Between the lymph node capsule and the cortex lies the subcapsular sinus. Within the framework of the trabeculae are smaller, sinus-like areas to which the subcapsular sinus connects. Lymph travels freely within the subcapsular sinus until it drains into these trabecular sinuses. These travel through the cortex of the lymph node into the sinuses of the medulla. Unlike other areas of the lymph node, the subcapsular sinus contains few lymphocytes.
The outer portion of the lymph node is the cortex. It possesses an outer area and a deeper paracortex. Within the superficial area are many follicles that consist of a multitude of rapidly dividing B cells. If an antigen challenges the B cells in the cortex, the B cells may begin to divide further and create plasma cells. The deeper paracortex contains mostly T cells that may interact with dendritic cells. The main purpose of dendritic cells is to process antigens and then use them to trigger T cells. The dendritic cells act as a bridge between the innate and adaptive immune systems.
Under the cortex lies the medulla, which contains many structures. This section holds large blood vessels, as well as the sinuses that facilitate the flow of lymph. It also contains the medullary cords, structures that consist of lymphatic tissue and contain a variety of B cells, macrophages, and plasma cells. Sinuses separate the medullary cords and connect to the efferent lymphatic vessel. Though most lymph nodes possess a single efferent lymphatic vessel, some may have two.
Lymph originates in the smallest of the lymphatic vessels, the lymphatic capillaries. It travels from these capillaries to the afferent lymphatic vessels, which connect to the subcapsular sinus, through which lymph enters the node. From there, the lymph flows into the trabecular sinuses of the cortex. It then travels into the medullary sinuses, where it collects for some time. Within the sinuses is another type of white blood cell: macrophages. The many macrophages can trap foreign pathogens and particles, filtering the lymph. After a short while, the lymph exits the lymph node via the efferent lymphatic vessels.
The lymph nodes’ most important function is filtering lymph to identify and then combat incoming infections. Lymphocytes circulate through the bloodstream before residing in the lymph nodes. Once B cells arrive in the lymph nodes, they produce antibodies that each target a single antigen. When they bind to these antigens, they stimulate an immune response. In some cases, this allows for the creation of plasma cells or memory cells that allow the body to fight future infections.
Conditions and diseases ranging from minor infections to serious cancers can inhibit or damage the lymph nodes. Often, this occurs through inflammation or enlargement. Inflammation of lymph nodes can cause pain and difficulty breathing and swallowing. If cancer is responsible, the lymph nodes can act as a key part of the diagnostic process. They can determine the presence of local disease, and inform physicians how far the cancer has progressed.
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