Lymph and Lymphatic System
o The lymphatic system is specialized vessels that perform the function of transportation of interstitial fluid to the intravascular space, to prevent fluid accumulation in the tissues.
o Interstitial fluid is body fluid between blood vessels and cells. Once this fluid enters lymphatic capillary it is called lymph. Lymph is a clear and colorless watery fluid that passes from the Lymph capillary and is released into the blood by way of the thoracic duct.
o Lymph resembles blood plasma contains white blood cells but lacks red blood cells. The lymphatic system involves Lymph capillaries, lymphatic vessels, lymphatic nodes, and collecting ducts.
o It performs an important function in fluid balance maintenance (removal of interstitial fluid from tissues), absorption of fats from the GI (gastrointestinal) tract, and assists the immune system in the trafficking of antigen and immune cells (Immune cells are sent to lymph nodes).
Diagrammatic Representation of Lymphatic System
o Lymph is transparent in most tissues but the quantitative composition of lymph is different in some tissues for example the lymph from the small intestine has more concentration of proteins and triglyceride fat globules (chylomicrons), which is dense and milky, indicating that chylomicrons produced from fat absorbed by the mucosal epithelium.
o The lymph from the small intestine and liver is called chyle.
o The interstitial fluid is leaked out of capillaries so it has plasma and proteins, cell debris, microorganisms, metabolic waster, and immune cells.
o It is taken up by the lymphatic capillaries and referred to as lymph.
o Pre-nodal lymph (lymph in capillaries) has the same composition as interstitial fluid: water, plasma proteins (albumins, fibrinogen, etc) and white blood cells, salt ions (sodium, calcium, chloride, magnesium), other molecules (sugars, fatty acids, signaling molecules and metabolic waste).
o Water is usually absorbed from lymph into the blood supply of lymph nodes therefore Post nodal lymph has a higher concentration of plasma protein and lymphocytes.
o Initial Lymphatic also called “lymphatic capillaries” “terminal lymphatics” are the smallest lymphatic vessels that act as an entry point for interstitial fluid.
o They have lymphatic endothelial cells layer tethered to surrounding tissues. The Interstitial fluid enters with the help of passive diffusion. The transient negative pressures and by the movements of surrounding tissues (muscles, arteries), Lymph propagate by this passive motion aided by intraluminal valves from capillary to vessels.
o Lymphatic capillaries link together to form lymphatic vessels. But once it enters lymphatic collecting vessels it’s not sufficient for the movement of a large amount of lymphatic fluid.
o Lymphatic collecting vessels have pericytes cells, they allow contraction in the peristaltic in a manner along the lymphatic vessel.
o The vessel’s valves stop backflow and A stretch in the wall of the collecting vessel causes the pericytes to stretch and helps in transporting lymph from one collecting duct to the next enters till it reaches the lymph node.
o At rest, the intrinsic pump, which consists of pericytes and valves, creates two-thirds of the lymphatic flow.
o Compression of skeletal muscles provides the remaining one-third of the 8L to 12L of lymphatic fluid.
o Lymph nodes are the bean-like structure that contains lymphocytes and protein, it filters lymph and monitor it for antigen and also helps in fighting these invaders, there are a total of 700 lymph nodes throughout the body with clusters located in the neck, armpit and groin region, nodes in the body about 1-2mm to 2cm in size.
o In non-inflammatory conditions, lymph flows the subcapsular sinus (around the outer portion of the lymphatic lobules).
o Lymph filters through series of lymphatic nodes and are then passed to collecting ducts. Collecting ducts are the thoracic duct (right lymphatic duct and left lymphatic duct) and it’s the main lymphatic vessel that drains lymph into the systemic venous system, it is connected to the subclavian vein.
o The leaking of interstitial fluid is necessary for the nourishment of tissues. The lymphatics serve as the entry point for this excess fluid to be returned to the central circulation.
o The lymphatic fluid is returned to the circulation for the very important function to maintain fluid balance ensures homeostasis of the tissue, prevents fluid accumulation avoids tissue from swelling and maintains an adequate volume of central circulation.
II. Immune System
o The Lymph system plays important role in facilitating adaptive immunity. After detection of an antigen by Dendritic immune cells along with antigen locates to nearest lymphatic vessel with the help of chemokines secreted by lymphatic endothelial cells.
o Then immune cells bind to a specific receptor on endothelial cells, which mobilize dendritic cells into the lymph nodes. Here antigens are presented to T cells present in the lymph node and lead to immune system activation.
Clinical Importance of Lymph
o Lymphedema is an incurable condition caused by Insufficient transport of lymph. This causes interstitial fluid accumulation in the tissues and causing inflammation, fatty tissue proliferation, and fibrosis. Primary lymphedema is caused by congenital diseases like Milroy disease and distichiasis.
o Whereas Secondary lymphedema is caused by injury or removal of lymph nodes for surgery or due to infection. Even though there is no cure available, the progression can be delayed with early diagnosis.
Lymphoma is cancer caused due by the uncontrolled proliferation and cell division of lymphocytes of lymph nodes, the cancer can block the lymphatic vessels and ducts or nodes and render the lymph flow, accumulation of fluid, and swelling of the tissues.
o There are different types of lymphoma including Hodgkin’s lymphoma and non-Hodgkin’s lymphoma in children.
iii. Cancer Metastasis
Metastasis is the spread of cancer cells from the primary tumor to another part of the body. The lymph nodes are said to be the initial site for metastasis. They can be easily taken up into the lymphatic system which leads to more spread and formation of secondary tumors in different body parts.
o The immune system is capable of identifying and removing these metastatic cells but it can get overwhelmed or subverted.
o Lymphadenopathy is swelling of the lymph nodes. It can be caused due to infection, inflammation, autoimmune disease, and malignancy. This is why detailed history and physical screening are performed.
o Infection involved strep throat, mononucleosis, HIV infection. lymphadenopathy caused by an inflammatory condition is also called Lymphadenitis. If remain untreated, it might exacerbate the course of an underlying disease which can eventually result in a weakened immune system, which can result in substantial morbidity and mortality.
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- The physiology of lymphocyte migration through the single lymph node in vivo. Semin Immunol . 1999 Apr;11(2):73-83.
- The lymph as a pool of self-antigens. Trends Immunol . 2011 Jan;32(1):6-11.