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Chapter 16
The Lymphatic System
o The lymphatic system is part of the immune system. The cardiovascular system plays a key role in bringing needed substances to cells and then removing the waste products that accumulate as a result of metabolism. This exchange of substances between blood and tissue fluid occurs in capillary beds. o Many additional substances that cannot enter or return through the capillary walls, including excess fluid and protein molecules, are returned to the blood as lymph. o The primary function of the lymphatic system is to transport lymph, a fluid containing infection- fighting white blood cells (lymphocytes), throughout the body. o The lymphatic system primarily consists of lymph , lymphatic vessels , which are connected to lymph nodes (where the lymph is filtered), the lymphatic organs like tonsils, adenoids, spleen and thymus.
Formation of Lymph : Lymph is a clear-to-white fluid formed in the tissue space contains a variety of substances, including proteins, salts, glucose, fats, water, and white blood cells (lymphocytes). Unlike blood, lymph does not normally contain any red blood cells. Lymph is formed in the tissue spaces. As blood circulates through the body, blood plasma leaks into tissue spaces through the thin walls of the capillaries. The portion of blood plasma that escapes is called interstitial or extracellular fluid. Although most of this fluid seeps immediately back into the bloodstream, a percentage of it, along with the particulate matter, is left behind which is then drained via the lymphatic vessels to the bloodstream as lymph. Lymphatic vessels — The lymphatic vessels often have a "beaded" appearance caused by the presence of valves that assist in maintaining a one-way flow of lymph. Unlike blood, lymph does not flow over and over again through vessels that form a circular route. Both veins and lymph vessels have one-way valve and carry fluid back to the heart. Lymphatic circulation: Lymph flowing through the lymphatic capillaries next moves into successively larger and larger vessels called lymphatic venules and veins and eventually empties into two terminal vessels called the right lymphatic duct and the thoracic duct , which empty their lymph into the blood in large veins in the neck region. Thoracic duct : Lymph from about three fourths of the body eventually drains into the thoracic duct, which is the largest lymphatic vessel in the body. Chyle , a specialized version of lymph, transports fats from your intestine to bloodstream. The special lymphatic vessels surrounding your intestine that collect chyle are called lacteals. Lacteals drain into a dilated sac - the cisterna chyli (meaning 'reservoir for chyle') - at the lower end of the thoracic duct. The thoracic duct then conveys the chyle to your bloodstream, where the fats it carries can be processed for energy or storage. Right lymphatic duct : Lymph from the right upper extremity and from the right side of the head, neck, and upper torso flows into the right lymphatic duct.
Lymphoid Organs Lymph nodes, the thymus, tonsils, and spleen are considered lymphoid organs because they contain lymphoid tissue. Lymphoid tissue is a mass of developing lymphocytes and related cells supported in a fine mesh of reticular fibers. Lymphoid organs are important structural components of the immune system because they provide immune defense and development of immune cells Lymph nodes o Filter lymph as lymph moves from its origin in the tissue spaces toward the thoracic or right lymphatic ducts and then into the venous blood o Located in clusters along the pathway of lymphatic vessels. Most lymph nodes occur in groups or clusters in certain areas. o Lymph nodes and other lymphoid organs have two functions that include defense and WBC (lymphocytes) formation DEFENSE FUNCTION: BIOLOGICAL FILTRATION Lymph nodes filters the lymph to remove bacteria, cancer cells and other abnormal cells which prevents local infections from spreading. Lymph enters the node through four afferent (Latin "to carry toward") lymphatic vessels. In passing through the node, lymph is filtered so that bacteria, cancer cells, and damaged tissue cells are removed and prevented from entering the blood and circulating all over the body. Lymph exits from the node through a single efferent (Latin "to carry away from") lymphatic vessel. Lymphadenitis —swelling and tenderness of lymph nodes. Metastasis (Spreading of cancer cells) : Cancer cells can easily move through lymphatic vessels to other parts of the body in a process called metastasis. Lymphoma — o Malignant tumor of lymph nodes o Two principal types: Hodgkin disease and non-Hodgkin lymphoma o Cause enlarged, painless lymph nodes followed by anemia, weight loss, weakness, fever, and spread
to other lymphoid tissues o When discovered early, lymphoma can be successfully treated with intensive radiation and chemotherapy. Lymphoma occurs more often in men than in women. Thymus o Located in the mediastinum, extending upward in the mid-line of the neck. It is composed of lymphocytes in a mesh like framework. o The thymus, also called the thymus gland, is largest at puberty and even then, weighs only about 35 or 40 g. o Produces T lymphocyte or T cells that leave the thymus and circulate to the spleen, tonsils, lymph nodes, and other lymphoid tissues. These T lymphocytes, or T cells, are critical to the functioning of the immune system. o Secretes hormones called thymosins , which influence T-cell development o The thymus tissue is then gradually replaced by fat and connective tissue, a process called involution. Tonsils o Composed of three masses of lymphoid tissue around the openings of the mouth and throat. Palatine tonsils (“the tonsils”)- located on each side of the throat Pharyngeal tonsils (also known as adenoids )- near the posterior opening of the nasal cavity Lingual tonsils- near the base of the tongue o They help protect us against bacteria that may invade tissues in the area around the openings between the nasal and oral cavities. o The tonsils serve as the first line of defense from the exterior and as such are subject to chronic infection, or tonsillitis. They may have to be removed surgically if antibiotic therapy is not successful at treating the chronic infection or if swelling impairs breathing Spleen o Largest lymphoid organ in body Located in upper left quadrant of abdomen lateral to the stomach. The spleen has a very rich blood supply and may contain more than 500 ml (about 1 pint) of blood. o If the spleen is damaged and bleeding, surgical removal, called a splenectomy , may be required to stop the loss of blood.
Active immunity occurs when an individual’s own immune system responds to an agent that produces an immune response, regardless of whether that agent was naturally or artificially encountered. Passive immunity results when immunity to a disease that has developed in another individual or animal is transferred to an individual who was not previously immune. For example, antibodies in a mother’s milk confer passive immunity to her nursing infant. Active immunity lasts much longer than passive immunity. Although passive immunity is temporary, it provides immediate protection Natural immunity —exposure to causative agent is not deliberate o Active—First time infection of any disease produces immunity. For example, first time measles or chicken pox infection gives rise to immunity against measles for life long. o Passive—immunity passes from mother to fetus through placenta or from mother to child through mother's milk. Artificial immunity —exposure to causative agent is deliberate o Active— Intentional exposure to the causative agent, such as a vaccination against polio, activates the immune system and thus confers immunity. o Passive—Injection of protective material (antibodies) developed in another individual's immune system and given to previously nonimmune individual. Example- antibodies for rabies and tetanus infection
Immune System Molecules
The immune system functions because of adequate amounts of defensive protein molecules and protective cells. The protein molecules critical to immune system functioning are called antibodies and complement proteins. Antibodies o Antibodies are protein compounds with specific combining sites and they are specific to an antigen. o Combining sites attach antibodies to specific antigens (foreign proteins), forming an antigen-antibody complex—called humoral, or antibody-mediated, immunity. o Antigens are often protein molecules imbedded in the surface membranes of threatening or diseased cells such as microorganisms or cancer cells o Antigen-antibody complexes may: Neutralize toxins Clump or agglutinate enemy cells. Then macrophages or the other phagocytes can rapidly destroy them by ingesting and digesting large numbers of them at one time. Another important function of antibodies is promotion and enhancement of phagocytosis. Certain antibody fractions help promote the attachment of phagocytic cells to the object they will engulf. They often stimulate the complement cascade. Complement Proteins o Group of proteins normally present in blood in inactive state. These proteins attach to antigens. The result is formation of highly specialized protein molecules that target foreign cells for destruction. This process is a rapid-fire cascade or sequence of events collectively called the complement cascade. o Complement cascade- Complement triggers a series (cascade) of reactions that produce tiny protein rings that create holes in the surface of a foreign cell (doughnut-shaped). o Ultimately causes cell lysis by permitting entry of water through a defect created in the plasma membrane of the foreign cell o Complement proteins also serve other roles in the immune system, such as attracting immune cells to a site of infection, activating immune cells, marking foreign cells for destruction, and increasing permeability of blood vessels. Complement proteins also play a vital role in producing the inflammatory response.
Immune System Cells (blood cells) The primary cells of the immune system include the following:
- Phagocytes: Neutrophils, Monocytes, Macrophages and Dendritic cells.
- Lymphocytes: T lymphocytes, B lymphocytes and Natural Killer cells. Phagocytes o Two important phagocytes are neutrophils and monocytes. They ingest and destroy foreign cells or other harmful substances via phagocytosis o Neutrophils are the most abundant immune cell in the body. Because neutrophils are needed only temporarily as phagocytes, they are short-lived in the tissues. o Monocytes are called aggressive phagocytes as they can migrate outside the blood circulation into the tissue space to attack bacteria. Once in the tissues, monocytes develop into phagocytic cells called macrophages. o Another type of phagocytic cell is called the dendritic cell (DC). These highly branched (dendrite “branch”) cells are produced in bone marrow and are released into the bloodstream. Some remain in the blood but many migrate to tissues in contact with the external environment—the skin, respiratory lining, digestive lining, and so on. Resident DCs in these barrier regions help protect us from threatening particles and cells. Lymphocytes: o The most numerous cells of the immune system after neutrophils are the lymphocytes. o Lymphocytes are responsible for antibody production and other immune mechanisms. Several million strong, lymphocytes continually patrol the body, searching out any enemy cells that may have entered or threatening virus-infected cells and cancer cells. o Lymphocytes densely populate the body's widely scattered lymph nodes and its other lymphatic tissues, especially the thymus gland in the chest and the spleen and the liver in the abdomen. Natural killer cells The natural killer (NK) cell is a type of lymphocyte that kills many types of tumor cells and cells infected by different kinds of viruses. NK cells recognize abnormal cells by using two different recognition receptors. One receptor acts as a “kill-activating” receptor and binds to any of several common surface molecules found on cells. Thus, it could potentially kill any cell in the body. However, the other receptor is the “kill-inhibiting” receptor. If it binds to a “self” antigen on a normal, healthy cell, then the killing action is “turned off.” Thus, only abnormal cells that lack normal self-antigens are killed. B Cells/B Lymphocytes Development of B Cells All lymphocytes that circulate in the tissues arise from primitive cells in the bone marrow called stem cells and go through two stages of development. The first stage of B-cell development: o Transformation of stem cells into immature B cells, occurs in the liver and bone marrow before birth but only in the bone marrow after birth. o After they mature , inactive B cells migrate chiefly to lymph nodes. Second Stage of B-Cell Development : o Inactive B cell develops into activated B cell. Initiated by inactive B cell’s contact with antigens , which bind to its (B cells) surface antibodies, plus signal chemicals from T cells. o Activated B cell, by dividing repeatedly , forms two clones of cells: Plasma (effector) cells and memory cells o Plasma cells secrete antibodies into blood; memory cells are stored in lymph nodes o If subsequent exposure to antigen that activated B cell occurs, memory cells can recognize the antigen that and then they become plasma cells and secrete antibodies. The antibody then binds with the
person's body and that are used by the immune system to identify components of "self." In autoimmunity, the immune system inappropriately attacks these antigens. o Systemic lupus erythematosus (SLE) is an example of autoimmune chronic inflammatory disease caused by numerous antibodies attacking a variety of tissues. The disease affects many tissues in the body: joints, blood vessels, kidneys, nervous system, and skin Alloimmunity o Alloimmunity is excessive reaction of the immune system to antigens from a different individual of the same species. May occur between mother and fetus during pregnancy (erythroblastosis fetalis) and tissue or organ transplants. Alloimmunity is also sometimes called isoimmunity o Tissue or organ transplants are medical procedures in which tissue from a donor is surgically grafted into the body. For example, skin grafts, blood transfusion, kidney transplant etc. Unfortunately, the immune system sometimes reacts against foreign antigens present in the grafted tissue, causing what is often called a rejection syndrome. The antigens most commonly involved in transplant rejection are called human lymphocyte antigens (HLAs). o There are two ways to prevent rejection syndrome. One strategy is called tissue typing, in which HLAs and other antigens of a potential donor and recipient are identified. If they match, tissue rejection is unlikely to occur. Another strategy is the use of immunosuppressive drugs in the recipient. Immunosuppressive drugs such as cyclosporine and prednisone suppress the immune system's ability to attack the foreign antigens in the donated tissue
Immune System Deficiency
Immune deficiency, or immunodeficiency, is the failure of immune system mechanisms in defending against pathogens. Immune system failure usually results from disruption of lymphocyte (B or T cell) function. The chief characteristic of immune deficiency is the development of unusual or recurring severe infections or cancer. There are two broad categories of immune deficiencies, based on the mechanism of lymphocyte dysfunction: congenital and acquired. Congenital immune deficiency, or immunodeficiency (rare) o Results from improper lymphocyte development before birth o Severe combined immune deficiency (SCID)—caused by disruption of stem cell development Acquired immune deficiency o Develops after birth o Acquired immunodeficiency syndrome (AIDS)—HIV damages the immune system by invading the T cells.
