A lymph node is a small ball or 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 immune 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.

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 of the lymph nodes contains lymphocytes, a type of white blood cell, 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 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 an antibody which will go out of the lymph node into circulation, seek, and target the pathogen producing the antigen by targeting it for destruction by other cells and complement. Other immune system cells will be made to fight the infection and "sent" to the lymph nodes. The increased numbers of immune system cells fighting the infection will make the node expand and become "swollen."

Dr Sarah Caroline Jarvis (born 3 December 1962) is a general practitioner working in Shepherd's Bush, London, England, and she also works in the mass media to promote health. She qualified as a medical doctor in July 1986 from University of Oxford. She is also the health and medical reporter for The One Show.