Lymph and Lymphatic Vessels

Lymph is a colorless fluid that contains white blood cells and is the interstitial fluid that fills the spaces between tissue cells. Lymph is distributed throughout the body via a network of lymphatic vessels. A lymphatic vessel is one of the vessels of the lymphatic system in the body. These vessels run between cells and intertwine with blood capillaries.

The spaces between cells (called interstitial spaces) fill with fluid containing nutrients and byproducts of metabolism. The lymphatic vessels absorb this fluid and drain it to collecting ducts. The ducts then return the nutrients to the bloodstream so they can be carried back to cells that need them. Lymphatic capillaries drain and return interstitial fluid to maintain homeostasis in the body.

In addition, lymph contains lymphocytes, which are white blood cells that function in immune response. The two main types are T cells and B cells. B cells and T cells recognize foreign bodies or pathogens, such as bacteria. Both B cells and T cells also produce memory cells and effector cells. A stem cell destined to become a B lymphocyte (B cell) originates and matures in the red bone marrow. B cells produce antibodies, which bind to pathogens such as bacteria and viruses and neutralize them. In contrast, a stem cell destined to become T lymphocyte (T cell) originates in red bone marrow and then migrates to the thymus, where the cell matures. T cells produce enzymes that kill infected cells, thereby destroying the pathogens.

B cells produce antibodies when they encounter a disease-causing pathogen. Antibodies are special molecules that bind to specific antigens. An antigen is any molecule that initiates an antibody-mediated immune response, such as foreign substances or organisms. B cells coat pathogens in antibodies, thus effectively neutralizing them so that they cannot continue to infect other cells.

T cells, however, do not produce antibodies. T cells may be of specific types, including T helper cells, cytotoxic T cells, and regulatory T cells. T helper cells release chemical signals called cytokines when a pathogen is present, recruiting other lymphocytes to the area. Cytotoxic T cells release enzymes that destroy infected cells. These cells work together, along with B cells, to identify, neutralize, and destroy pathogens all over the body. Regulatory T cells keep the body's immune response under control. Regulatory T cells produce inhibitory cytokines that interrupt pathways triggered by helper T cells in response to the body's own cells. In effect these cells act to modulate the immune system and prevent autoimmune response. Effector cells live only a short time and function as defense mechanisms against pathogens. The effector B cells, also called plasma cells, produce antibodies against infection. T cells become activated and trigger the production of two types of cells: helper T cells and cytotoxic T cells. A cytotoxic T cell attacks cancer cells, damaged cells, and cells that have contracted viruses. Helper T cells activate cytotoxic T cells and make B cells generate antibodies to fight infection.

Memory B cells form antibodies to antigens but do not coat the pathogen with them. Instead, memory B cells proliferate, producing more memory B cells with the same antibodies. Memory T cells "remember" antigens and proliferate without producing the cytokines that would call other lymphocytes to the area. These cells remain circulating throughout the lymphatic system. When the antigen is encountered again, each memory cell divides, producing effector cells (that attack the antigen) and more memory cells specific to that antigen. They release their antibodies and cytokines, speeding up the body's immune response and eliminating the threat before infection can set in. Memory B cells are the mechanism that allows vaccines to work.

Lymph is circulated throughout the body through lymphatic vessels. These vessels form capillaries that are much more permeable than blood capillaries and are blind-ended tubules, vessels closed at one end. A lymphatic capillary is a tiny, thin-walled vessel located in the spaces between cells (except in the central nervous system and nonvascular tissues) that serves to drain and process interstitial fluid. The lymphatic capillaries intertwine with the blood capillaries. Blood capillaries are found in all body tissues except epithelial tissue and connective tissues, such as ligaments. They absorb nutrients and fluid produced by the cells, enriching the lymph already circulating in the vessels. The lymphatic vessels carry this fluid to collecting ducts, which then drain it back into the circulatory system. The circulatory system can then distribute the nutrients back to the cells. This allows the nutrients absorbed by the capillaries to return to cells for use.

The lymphatic vessels may also take in pathogens such as bacteria or viruses. These pathogens flow with the lymph into the lymph nodes, where they encounter large concentrations of lymphocytes. The lymphocytes identify, neutralize, and destroy the pathogens, preventing them from spreading throughout the body. A lacteal is a lymphatic capillary located in the villi of the small intestine, which absorbs fats. Villi are tiny hair-like projections in the digestive tract. Nutrients such as sugars and proteins are absorbed into the blood in the small intestine and are filtered by the liver. Fats cannot be effectively filtered by the liver, and accumulation of fat in the liver can damage it. Instead, dietary fats and fat-soluble vitamins are first absorbed by the lacteals in the small intestine, which connect to the thoracic duct. The thoracic duct is a large lymphatic vessel in the chest, extending from the second lumbar vertebra to the neck and carrying four liters of lymph daily. The products of fat digestion cannot pass through narrow capillaries. They move through porous lacteals and enter the lymphatic system, bypassing the liver. In this way the lymphatic system protects the liver from damage. Although the heart pumps blood, it does not pump lymph. The lymphatic system depends on muscle and joint motion to generate movement through lymphatic vessels. Lymphatic capillaries intertwine with blood capillaries. Here, they absorb fluid and nutrients from the spaces between cells. The fluid and nutrients join the lymph as it flows through the lymphatic vessels. The lymphatic system moves in a singular direction. A system of valves in the afferent vessels prevents lymph from backing up as it passes through nodes. The circulatory system depends on arteries to carry blood away from the heart and veins to return blood back to the heart. The lymphatic system is a one-way path, in which flow begins with blind-ended capillaries.