The lymphatic system
The lymphatic system is part of the body's circulatory system. Body fluids are carried in vessels of the lymphatic system as well as in blood vessels. Together the blood vessels and lymph vessels form the bodies vascular, or vessel, system.
Lymph originates from blood plasma and tissue fluid that surrounds all body cells. It provides the medium through which diffusion of nutrients and gases occurs. Each day slightly more fluid filters out of the capillaries than is leabsorbed. Lymph and the valuable proteins it contains are collected in lymph capillaries, tiny vessels in almost every organ. The largest of the lymph vessels are lymph ducts, which empty into the two subclavian veins located in the neck. In this way, fluid and proteins are returned to the bloodstream.
The lymph system also helps protect the body against infection. Tiny bean-shaped organs called lymph nodes concentrated in the armpits, neck, and groin filter out such foreign matter as bacteria and viruses from lymph. Lymph tissue is also located in the tonsils, adenoids, spleen, thymus gland, digestive tract, and bone marrow. Lymph tissue also produces a type of white blood cell that helps the body fight disease.
If the lymphatic system malfunctions, excessive amounts of fluid collect in the body. This condition is known as edema. Generally, edema is a symptom of a more serious physical disorder.
Blood and tissue fluid carry nutrients to body cells. These substances are necessary for healthy cells. The blood also carries substances that defend the body against diseases.
Some defenses are called nonspecific defenses because they operate in the same way against all disease-causing microorganisms. Among nonspecific defenses are the skin and mucous membranes. They provide a mechanical barrier against pathogens which are disease-causing agents such as viruses and bacteria. If pathogens do enter the body, a type of white blood cell called a phagocyte engulfs and digests them. This process is known as phagocytosis. The dead bacteria and white blood cells may become pus. The presence of pus indicates an infection.
Virus-infected cells may also release the protein interferon. Interferon inactivates attacking viruses by preventing them from reproducing. All viruses stimulate the production of the same type of interferon, and interferon attacks all types of viruses.
The body also has specific defenses, by which it defends itself against specific pathogens. Body defenders constantly circulate in the bloodstream and tissue fluid, tracking down harmful microorganisms and diseases cells. When they locate their prey, they trigger a precisely targeted attack. These defenders are white blood cells called lymphocytes. The two main types of lymphocytes are B cells and T cells. These are complex white blood cells that stop the progression of diseases and infections.
Every body cell has molecules on its surface that identify it is as “self” – that is, as a part of the body. Foreign substances have surface molecules that tag them as “nonself”. If a surface molecule contacted by a lymphocyte is a “self” marker, nothing happens, and the lymphocyte moves on. If the molecule is a “nonself” marker, however, the body produces an attack on the foreign substance called an immune response. Any molecule that triggers an immune response is an antigen.
When a B cell identifies a “nonself” marker, it carries the pattern for that antigen to a lymph node. The B cell may then become a plasma cell. Plasma cells manufacture proteins that exactly fit the “nonself” surface marker of the antigen. These proteins are antibodies. Each antibody fits – or combats – just one specific antigen. This type of antibody, called a circulating antibody, moves through the body fluids, seeking out the appropriate antigen. When the antigen is located, the antibody hooks on and signals phagocytes to surround and destroy the antigen.
T cells do not produce circulating antibodies. They carry cellular antibodies on their surface. The cellular antibodies latch into an antigen and direct the action of phagocytes. T cells can recognize body cells that have been invaded by cancer and certain viruses. The cancerous cells register as “not quit self,” thus allowing T cells to launch a defense.
The body generally requires several days to form antibodies after the first attack by an antigen. Reaction to the first invasion is called the primary immune response. Future responses to the same antigen are rapid because of memory cells. Memory cells are B cells that carry, or “remember”, the antigen pattern.
They produce antibodies immediately if the antigen attacks again. Antibodies produced during such a secondary immune response are stronger and last longer than the original antibodies. Memory cells live for years. New memory cells are produced during each response. As a result, the response is faster and stronger each time. This process of warding off disease through antibodies is immunity. Immunity prevents a person from getting certain diseases, such as measles or chicken-pox, repeatedly.
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