A lymph node or lymph gland is an oval-shaped organ of the immune system, distributed widely throughout the body including the armpit and stomach and linked by lymphatic vessels. Lymph nodes are garrisons of B, T, and other immunity cells. Lymph nodes act as filters or traps for foreign particles and are important in the proper functioning of the immune system. They are packed tightly with the white blood cells called lymphocytes and macrophages.
Lymph nodes also have clinical significance. They become inflamed or enlarged in various conditions, which may range from trivial, such as a throat infection, to life-threatening such as cancers. In the latter, the condition of lymph nodes is so significant that it is used for cancer staging, which decides the treatment to be employed, and for determining the prognosis. When swollen, inflamed or enlarged, lymph nodes can be hard, firm or tender.
Lymph nodes can also be diagnosed by biopsy whenever they are inflamed. Certain diseases affect lymph nodes with characteristic consistency and location.
The lymph fluid inside the lymph nodes contains lymphocytes, a type of white blood cell, which continuously recirculates through the lymph nodes and the bloodstream. Molecules found on bacteria cell walls or chemical substances secreted from bacteria, called antigens, may be taken up by dedicated antigen presenting cells such as dendritic cells into the lymph system and then into lymph nodes. In response to the antigens, the lymphocytes in the lymph node make antibodies which will go out of the lymph node into circulation, seek, and target the pathogens producing the antigens by targeting them for destruction by other cells and complement. Other immune system cells will be made to fight the infection and "sent" to the lymph nodes.[clarification needed] The increased numbers of immune system cells fighting the infection will make the node expand and become "swollen."
The lymph node is surrounded by a fibrous capsule, and inside the lymph node the fibrous capsule extends to form trabeculae. The substance of the lymph node is divided into the outer cortex and the inner medulla surrounded by the cortex all around except at the hilum, where the medulla comes in direct contact with the surface.
Thin reticular fibers, elastin and reticular fibers form a supporting meshwork called reticular network (RN) inside the node, within which the white blood cells (WBCs), the most prominent ones being lymphocytes, are tightly packed as follicles in the cortex. Elsewhere, there are only occasional WBCs. The RN not only provides the structural support, but also the surface for adhesion of the dendritic cells, macrophages and lymphocytes. It allows exchange of material with blood through the high endothelial venules and provides the growth and regulatory factors necessary for activation and maturation of immune cells.
A lymph sinus is a channel within the lymph node lined by the endothelial cells along with fibroblastic reticular cells and allows smooth flow of lymph through them. Thus, the subcapsular sinus is a sinus immediately deep to the capsule, and its endothelium is continuous with that of the afferent lymph vessel. It is also continuous with similar sinuses flanking the trabeculae and within the cortex (cortical sinuses). The cortical sinuses and that flanking the trabeculae drain into the medullary sinuses, from where the lymph flows into the efferent lymph vessel.
Multiple afferent lymph vessels that branch and network extensively within the capsule bring lymph into the lymph node. This lymph enters the subcapsular sinus. The innermost lining of the afferent lymph vessels is continuous with the cells lining the lymph sinuses. The lymph gets slowly filtered through the substance of the lymph node and ultimately reaches the medulla. In its course it encounters the lymphocytes and may lead to their activation as a part of adaptive immune response.
The outer cortex consists mainly of the B cells arranged as follicles, which may develop a germinal center when challenged with an antigen, and the deeper cortex mainly consisting of the T cells. There is a zone known as the subcortical zone where T-cells (or cells that are mainly red) mainly interact with dendritic cells, and where the reticular network is dense. The predominant site within the lymph nodes which contain T cells & accessory cells is also known as paracortex (reticular network).
There are two named structures in the medulla:
- The medullary cords are cords of lymphatic tissue, and include plasma cells, macrophages, and B cells
- The medullary sinuses (or sinusoids) are vessel-like spaces separating the medullary cords. Lymph flows into the medullary sinuses from cortical sinuses, and into efferent lymphatic vessels. Medullary sinuses contain histiocytes (immobile macrophages) and reticular cells.
The medulla contains large blood vessels, sinuses and medullary cords that contain plasma cells secreting antibody.
Shape and size 
Human lymph nodes are bean-shaped and range in size from a few millimeters to about 1–2 cm in their normal state. They may become enlarged due to a tumor or infection, or inflamed due to leukemia. Lymphocytes, types of white blood cells, are located within honeycomb structures of the lymph nodes. Lymph nodes are enlarged when the body is infected, primarily because there is an elevated rate of trafficking of lymphocytes into the node from the blood, exceeding the rate of outflow from the node, and secondarily as a result of the activation and proliferation of antigen-specific T and B cells (clonal expansion). In some cases, they may feel enlarged because of a previous infection; although one may be healthy, one may still feel them residually enlarged.
Lymphatic circulation 
Lymph circulates to the lymph node via afferent lymphatic vessels and drains into the node just beneath the capsule in a space called the subcapsular sinus. The subcapsular sinus drains into trabecular sinuses and finally into medullary sinuses. The sinus space is criss-crossed by the pseudopods of macrophages, which act to trap foreign particles and filter the lymph. The medullary sinuses converge at the hilum and lymph then leaves the lymph node via the efferent lymphatic vessel towards either a more central lymph node or ultimately for drainage into a central venous subclavian blood vessel, most via the postcapillary venules, and cross its wall by the process of diapedesis.
- The B cells migrate to the nodular cortex and medulla.
- The T cells migrate to the deep cortex ("paracortex").
When a lymphocyte recognizes an antigen, B cells become activated and migrate to germinal centers (by definition, a "secondary nodule" has a germinal center, while a "primary nodule" does not). When antibody-producing plasma cells are formed, they migrate to the medullary cords. Stimulation of the lymphocytes by antigens can accelerate the migration process to about 10 times normal, resulting in characteristic swelling of the lymph nodes.
Humans have approximately 500-600 lymph nodes distributed throughout the body, with clusters found in the underarms, groin, neck, chest, and abdomen.
Lymph nodes of the head and neck 
Lymph nodes of the thorax 
- Lymph nodes of the lungs: The lymph is drained from the lung tissue through subsegmental, segmental, lobar and interlobar lymph nodes to the hilar lymph nodes, which are located around the hilum (the pedicle, which attaches the lung to the mediastinal structures, containing the pulmonary artery, the pulmonary veins, the main bronchus for each side, some vegetative nerves and the lymphatics) of each lung. The lymph flows subsequently to the mediastinal lymph nodes.
- Mediastinal lymph nodes: They consist of several lymph node groups, especially along the trachea (5 groups), along the esophagus and between the lung and the diaphragm. In the mediastinal lymph nodes arises lymphatic ducts, which drains the lymph to the left subclavian vein (to the venous angle in the confluence of the subclavian and deep jugular veins).
The mediastinal lymph nodes along the esophagus are in tight connection with the abdominal lymph nodes along the esophagus and the stomach. That fact facilitates spreading of tumors cells through these lymphatics in cases of cancers of the stomach and particularly of the esophagus. Through the mediastinum, the main lymphatic drainage from the abdominal organs goes via the thoracic duct (ductus thoracicus), which drains majority of the lymph from the abdomen to the above mentioned left venous angle.
Lymph nodes of the arm 
These drain the whole of the arm, and are divided into two groups, superficial and deep. The superficial nodes are supplied by lymphatics that are present throughout the arm, but are particularly rich on the palm and flexor aspects of the digits.
- Superficial lymph nodes of the arm:
- Deep lymph nodes of the arm: These comprise the axillary nodes, which are 20-30 individual nodes and can be subdivided into:
- Lateral nodes
- Anterior or pectoral nodes
- Posterior or subscapular nodes
- Central or intermediate nodes
- Medial or subclavicular nodes
Lower limbs 
Lymphadenopathy is a term meaning "disease of the lymph nodes." It is, however, almost synonymously used with "swollen/enlarged lymph nodes." In this case, the lymph nodes are palpable, and is a sign of various infections and diseases.
Additional images 
- Warwick, Roger; Peter L. Williams (1973) . "Angiology (Chapter 6)". Gray's anatomy. illustrated by Richard E. M. Moore (Thirty-fifth ed.). London: Longman. pp. 588–785.
- Kaldjian, Eric P.; J. Elizabeth Gretz, Arthur O. Anderson, Yinghui Shi and Stephen Shaw (October 2001). "Spatial and molecular organization of lymph node T cell cortex: a labyrinthine cavity bounded by an epithelium-like monolayer of fibroblastic reticular cells anchored to basement membrane-like extracellular matrix". International Immunology (Oxford Journals) 13 (10): 1243–1253. doi:10.1093/intimm/13.10.1243. PMID 11581169. Retrieved 2008-07-11.
- Katakai, Tomoya; Takahiro Hara, Hiroyuki Gonda1, Manabu Sugai and Akira Shimizu (2004-07-05). "A novel reticular stromal structure in lymph node cortex: an immuno-platform for interactions among dendritic cells, T cells and B cells". International Immunology 16 (8): 1133–1142. doi:10.1093/intimm/dxh113. PMID 15237106. Retrieved 2008-07-11.
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