Small, rounded mass of lymphoid tissue contained in connective tissue. They occur all along lymphatic vessels, with clusters in certain areas (e.g., neck, groin, armpits). They filter bacteria and other foreign materials out of lymph and expose them to lymphocytes and macrophages that can engulf them; these cells multiply in response to accumulation of such materials, which is why lymph nodes swell during infections. The nodes also produce lymphocytes and antibodies, to be carried by lymph throughout the lymphatic system. In Hodgkin disease and other lymphomas, malignant lymph cells proliferate, causing lymph node enlargement. Other cancers often invade lymphatic vessels, which can carry cells from the tumour to lymph nodes, where they are trapped and grow into secondary tumours. Lymph nodes are therefore removed in cancer surgery to detect or prevent tumour spread.

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Pale fluid that bathes tissues, maintaining fluid balance and removing bacteria. It enters the blood system at a vein under the collarbone that it reaches via channels and ducts, being driven through them mainly by surrounding muscle activity. The lymphatic organs (spleen and thymus) and lymph nodes filter out bacteria and other particles the lymph takes up from body tissues. Lymph contains lymphocytes and macrophages, the primary cells of the immune system. Seealso lymphatic system.

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Lymph is the fluid that is formed as the interstitial fluid enters the lymph vessels by filtration. The lymph then travels to at least one lymph node before emptying ultimately into the right or the left subclavian vein, where it mixes back with blood.

Composition of lymph

Lymph has a composition comparable to that of plasma, but it is different in various parts of the body depending upon the tissue drained. In particular, the lymph that leaves a lymph node is richer in lymphocytes. Likewise, the lymph formed in the digestive system called chyle is rich in triglycerides (fat), and looks white.

Formation of lymph

Blood supplies nutrients, and important metabolites to the tissues, and collects back the waste products that they produce, which requires exchange of respective constituents between the blood and tissues. However, this exchange is not direct, and is effected through an intermediary called interstitial fluid or tissue fluid that the blood forms. Interstitial fluid (ISF) is the fluid that occupies the spaces between the cells and acts as their immediate environment. As the blood and the surrounding cells continually add and remove substances from the ISF, its composition keeps on changing. Water and solutes can freely pass (diffuse) between the ISF and blood, and thus both are in dynamic equilibrium with each other; exchange between the two fluids occurs across the walls of small blood vessels called capillaries.

ISF forms at the arterial (coming from the heart) end of the capillaries because of higher pressure of blood, and most of it returns to its venous ends and venules; the rest (10—20%) enters the lymph capillaries as lymph. Thus, lymph when formed is a watery clear liquid with the same composition as the ISF. However, as it flows through the lymph nodes it comes in contact with blood, and tends to accumulate more cells (particularly, lymphocytes) and proteins.

The two primary lymph systems are the thymus gland and the bone marrow, where the immune cells form or mature. The secondary lymph system is made up of encapsulated and unencapsulated diffuse lymphoid tissue. The encapsulated tissue includes the spleen and the lymph nodes. The unencasulated tissue includes the gut associated lymph tissues and the tonsils.

Lymphatic circulation

Tubular vessels transport back lymph to the blood ultimately replacing the volume lost from the blood during the formation of the interstitial fluid. These channels are the lymphatic channels or simply called lymphatics.

Unlike the cardiovascular system, the lymphatic system is not closed and has no central pump. Lymph movement occurs despite low pressure due to peristalsis (propulsion of the lymph due to alternate contraction and relaxation of smooth muscle), valves, and compression during contraction of adjacent skeletal muscle and arterial pulsation.

Usually, the lymph that enters the lymph vessels from the interstitial space, does not leak back because of presence of valves. But, in case of blockage of free flow, when excessive hydrostatic pressure develops within the lymph vessels, some fluid can leak back and contribute to formation of edema.

See also

• Lymphatic system
• Lymphedema

References