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Lymphatic system

Blood in the heart is highly pressurized, but most of that pressure is lost as blood flows through arteries and then arterioles. At the capillary level, most remaining pressure is lost as blood leaks out into tissues. This leaked fluid is unable to go against the pressure gradient back into the bloodstream, so instead diffuses into lymph capillaries of the lymphoid system. Once fluid has been absorbed into the lymphatic system it is called lymph. Lymph has almost the same composition as blood.

The lymphatic system's primary function is to keep the blood circulatory system a closed system that allows transfer only out of vessels. By definition, all fluids outside of cells and not in the bloodstream are by definition lymph. Once lymph diffuses into lymph capillaries from capillary tissues, it coalesces into lymph vessels of increasing size. These vessels form into ducts which, at the neck, eventually dump lymph back into the bloodstream.

The lymphatic system's secondary function is to completely drain the body of fluid, and screen this lymph for invaders. The lymphatic system has high concentrations of white blood cells to mount a response, including lymphocytes, macrophages and dendritic cells -- but no granulocytes.

The lymphatic system has no central pump; however, its vessels have one-way valves through which lymph flows. This flow is driven by skeletal muscles milk the lymph along as the body moves. Lymph passes through progressively larger vessels, eventually arriving in the right lymphatic duct (for fluid from the right upper body) or the thoracic duct (for the rest of the body). These ducts drain into the circulatory system in the neck, at the right and left subclavian veins.

Lymph vessels are present in gastrointestinal lining, absorbing fats and dumping them into the thoracic duct. The thoracic duct then drains these fats directly into the blood circulatory system. After passing through the stomach, food enters the small intestine. Chyle is the lipid-rich lymph drawn from the small intestine. The liver processes all nutrients present in the circulatory system, including the chyle that has been dumped into the bloodstream.

If you are learning immunology, then please read about lymph nodes before continuing.

Primary lymphoid organs

Bone marrow and the thymus are the primary lymphatic organs (aka central lymphoid tissues). Lymphocytes are produced by stem cells in the bone marrow and then migrate to either the thymus or bone marrow where they mature. T-lymphocytes undergo maturation in the thymus, and B-lymphocytes undergo maturation in the bone marrow. After maturation, both B- and T-lymphocytes circulate in the lymph and accumulate in secondary lymphoid organs, where they await recognition of antigens.

Secondary lymphoid organs

The spleen, lymph nodes, and accessory lymphoid tissue are the secondary lymphoid organs (aka peripheral lymphoid tissues). These organs contain a scaffolding that support circulating B- and T-lymphocytes and other immune cells like macrophages and dendritic cells. When micro-organisms invade the body or the body encounters other antigens (such as pollen), the antigens are transported from the tissue to the lymph. The lymph is carried in the lymph vessels to regional lymph nodes. In the lymph nodes, the macrophages and dendritic cells phagocytose the antigens, process them, and present the antigens to lymphocytes, which can then start producing antibodies or serve as memory cells to recognize the antigens again in the future.

The spleen contains lymphocytes that filter the blood stream rather than the lymphatics. Thus, the spleen has importance in fighting infections that have invaded the blood.

Mucosa-associated lymphoid tissue is specialised lymphoid tissue associated with the mucosa of a number of organs.

Lymph node

Lymph enters a lymph node via an afferent vessel, and leaves via efferent vessels.

Inside the lymph node, lymph percolates through various layers of tissue. The lymph then continues on its path into the blood circulatory system.

Reticular CellsReticular cells are found throughout the node and help trap antigens passing through; also, reticular cells are fibrous and help maintain the structure of the node. Reticular cells are much like dendritic cells, except reticular cells are phagocytic.
CortexThe cortex contains mostly virgin B cells, with dense areas known as follicles. Follicular dendritic cells trap antigen, with the follicles eventually becoming germinal centers as the host becomes more immunologically experienced. Memory cells are in mantles of secondary follicles.
ParacortexThe paracortex is mostly comprised of T cells, which move up to the cortex to support B cells. Also, T cells may get picked up by lymph to eventually patrol the bloodstream.

The medulla is the last stop for lymph flowing through a lymph node; the medulla empties into the efferent lymph vessel. Reticular cells condense in the medulla to form medullary chords. Although the medulla contains less cells than the cortex and paracortex, it is the location where B cells differentiate to become plasma cells.

In B cell differentiation, B cells begin producing antibodies that flow through the lymph to the entire body. This helps create the body’s immune memory, with large amounts of antibody loaded in the bone marrow and at germinal centers.

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