Lymph nodes are small oval-shaped balls of lymphatic tissue, distributed widely throughout the body and linked by lymphatic vessels. Lymph nodes are repositories of B cells, T cells, and other immune cells. They act as filters for foreign particles in the body and are important in the proper functioning of the immune system.
Lymph nodes are found throughout the body, and are typically 1-2 cm long. Humans have approximately 500-600 lymph nodes, with clusters found in the underarms, groin, neck, chest, and abdomen . Each lymph node is surrounded by a fibrous capsule, which encircles the internal cortex and medulla . The cortex is mainly composed of clusters of B and T cells. The medulla contains plasma cells, macrophages, and B cells, as well as sinuses, which are vessel-like spaces that the lymph flows into.
Lymph fluid flows into and out of the lymph nodes via the lymphatic vessels, a network of valved vessels that are similar in structure to cardiovascular veins. Each lymph node has an afferent lymph vessel, which directs lymph into the node, and an efferent lymph vessel, which directs lymph out of the node at the concave side of the node, known as the hilum.
Lymph nodes contain lymphocytes, a group of white blood cells that includes B cells, T cells and natural killer cells, which are continuously recirculated 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 cells into the lymph system and then into lymph nodes. In response to the antigens, the lymphocytes in the lymph node make an antibody which will go out of the lymph node into circulation to seek out the pathogen that is producing the antigen, targeting it for destruction by other immune cells. Additional immune system cells will be made to fight the infection and are sent to the lymph nodes. The increased numbers of immune system cells fighting the infection will make the node expand and become swollen.
Lymphadenopathy describes the condition when lymph nodes are swollen. The changes observed in lymph nodes 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 situation, 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.