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Chapter16 - Lympatic and Immune Systems
- 1. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company Chapter 16 TThhee LLyymmpphhaattiicc aanndd IImmmmuunnee SSyysstteemmss
- 2. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company
- 3. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh FFuunnccttiioonnss ooff tthhee llyymmpphhaattiicc ssyysstteemm Maintenance of fluid balance Absorption of fats Immunity Copyright © 2012 F.A. Davis Company
- 4. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company LLyymmpphhaattiicc vveesssseellss
- 5. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Which is not a function of the lymphatic system? A.Maintenance of fluid balance B.Absorption of fats C.Maintenance of body temperature D.Immunity Copyright © 2012 F.A. Davis Company
- 6. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Correct answer: C Rationale: The lymphatic system plays no role in the maintenance of body temperature. Copyright © 2012 F.A. Davis Company
- 7. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company Lymph node View animation of flow Lymphatic trunk
- 8. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company CCoolllleeccttiinngg dduuccttss
- 9. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company LLyymmpphhaattiicc oorrggaannss Red bone marrow Thymus Lymph nodes Tonsils Spleen
- 10. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh What forms the basis of lymphatic fluid? A.Tissue fluid left behind after capillary exchange B.Secretions of lymph nodes C.Secretions from the cells lining lymphatic capillaries D.Plasma Copyright © 2012 F.A. Davis Company
- 11. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Correct answer: A Rationale: Lymphatic fluid results from tissue fluid left behind after capillary exchange. Copyright © 2012 F.A. Davis Company
- 12. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company TThhyymmuuss View animation of maturing
- 13. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company LLyymmpphh nnooddeess Fibrous capsule Trabeculae Cortical nodules Germinal centers Sinuses Afferent lymphatic vessel Efferent lymphatic vessel
- 14. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company TToonnssiillss Pharyngeal tonsil (adenoids) Palatine tonsils Lingual tonsils
- 15. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company SSpplleeeenn White pulp Red pulp
- 16. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company SSpplleeeenn ffuunnccttiioonnss Immunity Destruction of old red blood cells Blood storage Hematopoiesis
- 17. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh In which lymphatic organ do T cells mature? A.Red bone marrow B.Tonsils C.Spleen D.Thymus Copyright © 2012 F.A. Davis Company
- 18. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Correct answer: D Rationale: T cells evolve from stem cells in red bone marrow, but they leave the bone marrow before maturity. Both the tonsils and spleen are lymphatic organs, but they play no role in T cell maturity. Copyright © 2012 F.A. Davis Company
- 19. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company IImmmmuunnee ssyysstteemm TThhrreeee lliinneess ooff ddeeffeennssee First: External barriers Second: Nonspecific immunity Third: Specific immunity
- 20. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company NNoonnssppeecciiffiicc iimmmmuunniittyy Phagocytosis Antimicrobial proteins Natural killer cells Inflammation Fever
- 21. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company PPhhaaggooccyyttoossiiss View animation
- 22. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company TTyyppeess ooff pphhaaggooccyytteess View animation of chemotaxis
- 23. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company AAnnttiimmiiccrroobbiiaall pprrootteeiinnss Interferons Complement system
- 24. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company IInntteerrffeerroonnss
- 25. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Which is an important white blood cell that travels throughout the bloodstream seeking out bacteria? A.Macrophage B.Phagocyte C.Neutrophil D.Phagosome Copyright © 2012 F.A. Davis Company
- 26. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Correct answer: C Rationale: Macrophages are important white blood cells that remain fixed in strategic areas. Phagocytes are white blood cells that ingest microorganisms. A phagosome is the sac formed around a microorganism when it’s ingested by a phagocyte. Copyright © 2012 F.A. Davis Company
- 27. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company CCoommpplleemmeenntt ssyysstteemm View animation of complement system
- 28. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company NNaattuurraall kkiilllleerr cceellllss Unique group of lymphocytes Roam the body Use several methods to destroy cells
- 29. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company IInnffllaammmmaattiioonn View animation of inflammatory response
- 30. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company FFeevveerr 1. Neutrophils and macrophages secrete a pyrogen. 2. Hypothalamus secretes prostaglandin E (PGE). 3. PGE resets body’s temperature set point. 4. The temperature rises. 5. After pathogen is gone, phagocytes stop producing the pyrogen. 6. Temperature set point returns to normal.
- 31. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Which statements about nonspecific immunity is not true? A.It is present from birth. B.It offers protection against a wide variety of threats. C.It uses mechanisms such as inflammation, fever, and phagocytic white blood cells. D.It retains a memory of a pathogen after defeating it. Copyright © 2012 F.A. Davis Company
- 32. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Correct answer: D Rationale: Nonspecific immunity does not retain a memory of a pathogen. Copyright © 2012 F.A. Davis Company
- 33. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company SSppeecciiffiicc iimmmmuunniittyy Cellular (cell-mediated) immunity Humoral (antibody-mediated) immunity
- 34. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company LLyymmpphhooccyytteess
- 35. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company AAnnttiibbooddiieess
- 36. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company TTyyppeess ooff iimmmmuunniittyy AAccttiivvee: When the body makes its own antibodies or T cells PPaassssiivvee: When immunity results from an injection of antibodies from another person or animal
- 37. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company CCllaasssseess ooff iimmmmuunniittyy Natural active Artificial active Natural passive Artificial passive
- 38. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company CCeelllluullaarr iimmmmuunniittyy Destroys pathogens within a cell Uses three classes of T cells: Cytotoxic T cells Helper T cells Memory T cells
- 39. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh View animation of cellular immunity Copyright © 2012 F.A. Davis Company
- 40. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company HHuummoorraall iimmmmuunniittyy Focuses on pathogens outside the cell Uses antibodies to mark antigen for later destruction
- 41. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh View animation of humoral immunity Copyright © 2012 F.A. Davis Company
- 42. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Copyright © 2012 F.A. Davis Company AAlllleerrggiicc rreeaaccttiioonn Upon first exposure, body produces IgE. Person is now sensitized. Upon subsequent exposure, allergen binds to antibodies. Mast cells release histamine. View animation of allergic reaction
- 43. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Which type of immunity uses T cells to destroy pathogens within a cell? A.Cellular immunity B.Humoral immunity C.Passive immunity D.Nonspecific immunity Copyright © 2012 F.A. Davis Company
- 44. UUnnddeerrssttaannddiinngg AAnnaattoommyy && PPhhyyssiioollooggyy AA VViissuuaall,, IInntteerraaccttiivvee AApppprrooaacchh Correct answer: A Rationale: Humoral immunity focuses on pathogens outside a cell. Passive immunity results after receiving antibodies from another person or animal. Nonspecific immunity uses a variety of mechanisms but does not use T cells. Copyright © 2012 F.A. Davis Company
Editor's Notes
- The lymphatic system consists of lymphatic vessels, lymph (the fluid within the vessels), lymphatic tissue, and lymphatic organs. T The vessels of the lymphatic system cover the body in much the same way as blood vessels. Unlike blood vessels, lymphatic vessels carry fluid in one direction only: away from the tissues. The tissues and organs of the lymphatic system—the lymph nodes, thymus, tonsils, spleen, and red bone marrow—produce immune cells.
- Maintenance of fluid balance Fluid continually seeps out of capillaries into surrounding tissues; capillaries reabsorb about 85% of the fluid, leaving about 15% behind. Over the course of a day, the remaining fluid would total as much as 4 liters—enough to cause massive swelling and even death. One of the roles of the lymphatic system is to absorb this fluid and return it to the bloodstream. Absorption of fats Specialized lymphatic vessels in the small intestines absorb fats and fat-soluble vitamins. Immunity Lymph nodes and other lymphatic organs filter lymph (the fluid inside the lymphatic vessels) to remove microorganisms and foreign particles.
- Lymphatic vessels have thin walls and valves to prevent backflow. Lymphatic vessel walls are formed by a thin layer of epithelial cells. Unlike the cells in veins (which are tightly joined), the cells that form lymphatic vessel walls overlap loosely, allowing gaps to exist between the cells. Fluid enters lymphatic vessels between the overlapping epithelial cells. Valves prevent backflow, ensuring that lymph moves steadily away from the tissues and toward the heart. Protein filaments anchor the capillaries to surrounding cells, which prevent the vessel from collapsing.
- Lymphatic vessels originate in tissue spaces as microscopic, blind-ended sacs within a bed of blood capillaries. Tissue fluid (the fluid left behind after capillary exchange) flows into the vessels through gaps between the cells. Bacteria, lymphocytes, and other cells flow in with the fluid. The vessels converge to form larger and larger vessels. Periodically, the vessels empty into lymph nodes, where immune cells phagocytize bacteria. The vessels continue to merge, eventually forming still larger lymphatic trunks, which drain major regions of the body. The lymphatic trunks converge to form two collecting ducts (one near the right subclavian vein and one near the left subclavian). Lymph joins the bloodstream when the collecting ducts merge into the subclavian veins. Fluid moves passively, aided primarily by rhythmic contractions of the lymphatic vessels. Flow is aided further by contraction of skeletal muscles; also, respiration causes pressure changes that help propel lymph from the abdominal to the thoracic cavity.
- The lymphatic system has two collecting ducts: the right lymphatic duct and the thoracic duct. The right lymphatic duct drains lymph for the upper right quadrant of the body into the right subclavian vein. The thoracic duct (which originates at a dilated portion of a lymphatic vessel in the abdomen called the cisterna chyli) drains lymph from the rest of the body into the left subclavian vein.
- Red bone marrow and the thymus are primary lymphatic organs; they provide a location for B and T lymphocytes to mature. Lymph nodes, tonsils, and spleen are secondary lymphatic organs; they contain lymphocytes that have matured in red bone marrow or the thymus. Patches of specialized tissue containing lymphocytes exist throughout the body; also, passages that open to the outside of the body (such as the respiratory, digestive, urinary, and reproductive tracts) contain a scattering of lymphocytes in their mucosa linings. Lymphatic tissue exists in masses called lymphatic nodules; such as Peyer’s patches, which are lymphatic nodules in the small intestines.
- The thymus is located in the mediastinum. It is quite large in children but begins to shrink at age 14. By adulthood, it is a fraction of its former size. The thymus is divided into lobules that extend inward from a fibrous outer capsule. Each lobule consists of a dense outer cortex and a less dense medulla filled with T lymphocytes. Immature T lymphocytes travel from red bone marrow to the outer cortex of the thymus. Inside the thymus, the cells are protected from antigens in the blood, giving them a chance to divide and mature. The developing T lymphocytes migrate toward the inner medulla. As they do, they encounter other lymphoid cells (such as macrophages and dendritic cells). This process “trains” the new lymphocytes to distinguish between the cells of its host body and foreign cells. Once the training is complete, the lymphocytes are released into the bloodstream.
- The body contains hundreds of lymph nodes. Shaped like a bean, some nodes are tiny: only 1/25” (1 mm) long; others are more than an inch (25 mm). Lymph passes through multiple lymph nodes; flow slows to a trickle as the lymph node removes pathogens and other foreign material. Lymph nodes also serve as sites for final maturation of some types of lymphocytes and monocytes. A fibrous capsule encloses each lymph node. Connective tissue called trabeculae extend into the node, dividing it into compartments; these compartments, called cortical nodules, are filled with lymphocytes. A less dense area at the center of the compartments (germinal centers) form and release lymphocytes when an infection is present. Sinuses lined with macrophages capable of phagocytosis separate the compartments. Lymph slowly flows through these sinuses in the process of being filtered. Several afferent lymphatic vessels channel fluid into a node. After slowly filtering through the node, lymph leaves through a single efferent lymphatic vessel.
- Tonsils are masses of lymphoid tissue. They form a circle at the back of the throat where they guard against pathogens entering the body through the nose or throat. There are three sets of tonsils: A single pharyngeal tonsil (also called adenoids) sits on the wall of the pharynx, just behind the nasal cavity. A pair of palatine tonsils lies in the posterior of the oral cavity. Numerous lingual tonsils are concentrated in patches on each side of the base of the tongue. Palatine tonsils are the largest; they are the most prone to becoming infected.
- The spleen (which is about the size of a fist) is the body’s largest lymphatic organ. Just like lymph nodes, the spleen is surrounded by a fibrous capsule; inward extensions of the capsule divide the spleen into compartments. The spleen contains two types of tissue: red pulp and white pulp. White pulp contains compact masses of lymphocytes; it surrounds the arteries leading into each compartment. Red pulp consists of a network of erythrocyte-filled sinuses supported by a framework of reticular fibers and phagocytic cells. Blood collects in the venous sinuses after passing through the reticular fibers; it then returns to the heart through the veins.
- Immunity: Lymphocytes and macrophages in the white pulp screen passing blood for foreign antigens; also, phagocytic cells in the sinuses ingest and destroy microorganisms. Destruction of old red blood cells (RBCs): Macrophages in the sinuses remove and digest worn-out RBCs and imperfect platelets. The macrophages also recycle hemoglobin from destroyed RBCs, returning the iron and globin to the bone marrow and liver for later use. Blood storage: The spleen stores 20% to 30% of the body’s platelets. Hematopoiesis: The spleen produces RBCs in the fetus.
- First line of defense: The skin and mucous membranes keep most pathogens at bay. The skin has an acid mantle that inhibits bacterial growth. Mucus, tears, and saliva contain lysozyme, an enzyme that destroys bacteria. Second line of defense: If a pathogen penetrates the first line of defense, the body launches nonspecific immunity (also called innate immunity because the mechanisms are present from birth). This involves mechanisms aimed at a wide variety of threats, such as the production of phagocytic white blood cells and triggering inflammation and fever. Third line of defense: Called specific immunity, this occurs when the body retains a memory of a pathogen after defeating it. If exposed to the same pathogen again, the body can recognize it and target a response at this one specific invader.
- All of these mechanisms are a part of nonspecific immunity. They are aimed at a broad range of attackers.
- Phagocytes are cells whose sole job is to ingest and destroy microorganisms and other small particles. When a phagocyte encounters a microorganism, it sends out membrane projections called pseudopods (or “false feet”). The pseudopods envelop the organism, forming a complete sac called a phagosome. The phagosome travels to the interior of the cell and fuses with a lysosome, which contains digestive enzymes. The digestive enzymes from the lysosome destroy the microorganism. The phagocyte expels the waste products.
- The most important phagocytes are neutrophils and macrophages. Neutrophils roam the body; most macrophages remain fixed within strategic areas. Neutrophils are summoned to an infection by a chemical released from inflamed cells (chemotaxis). The neutrophils anchor themselves to the inside of the blood capillary. They then dissolve a portion of the basement membrane, which allows them to squeeze out of the vessel (a process called diapedesis) and enter the inflamed tissue. Macrophages congregate in areas where microbial invasion is likely to occur: the alveolus of the lungs, the liver, nerve tissue, bone, and the spleen.
- When a virus infects a cell, the cell produces interferon, which it releases to nearby cells. The interferon binds to surface receptors on neighboring cells. This triggers the production of enzymes within the cells that would prevent the virus from replicating if it managed to invade.
- More than 20 different proteins (called complement) circulate in the bloodstream in an inactive form. A bacteria, or antibodies against the bacteria, activate the complement. A complement reaction continues as a cascade of chemical reactions, with one complement protein activating the next. The final five proteins (called the membrane attack complex) embed themselves into the bacterium’s plasma membrane in ringlike circles, punching a hole in the bacterium. Fluid and sodium (Na+) rush into the bacterium through the openings, and the bacterium swells and bursts. Complement also coats pathogens, making them attractive to phagocytes, and stimulate inflammation (which summons neutrophils through chemotaxis).
- Natural killer (NK) cells recognize and destroy any foreign cells, including cancer cells, virus-infected cells, bacteria—as well as the cells of transplanted organs and tissues. NK cells use several methods to destroy the cells. Most of them involve the secretion of chemicals that causes the cell to die and break apart (lysis).
- Injured cells secrete chemicals such as histamine that dilate blood vessels in the area. Blood rushes in, bringing leukocytes. The same chemicals trigger vasodilation and cause cells in the capillary wall to separate. Fluid, leukocytes, plasma proteins, antibodies, clotting factors, and complement leak out. Fibrinogen forms a sticky clot to keep the infection from spreading. Neutrophils (which have been drawn to the area by chemotaxis) phagocytize pathogens. They also secrete chemicals (cytokines) to summon other neutrophils and macrophages. Macrophages destroy bacteria and clean up the area by engulfing damaged cells and dead neutrophils.
- Fever is beneficial: It promotes the activity of interferon and inhibits the reproduction of bacteria and viruses. The sequence of events in a fever is as follows: As neutrophils and macrophages phagocytize bacteria, they secrete a substance called a pyrogen. The pyrogen stimulates the anterior hypothalamus to secrete PGE. PGE resets the body’s set point for temperature. (For example, it may raise it from a normal of 98.6° F [37°C] to 102° F [39°C].) When the set point rises, the body needs to generate heat; it does this through shivering and constricting blood vessels in the skin. The result is chills and cold, clammy skin. The temperature rises until it reaches its new set point; it stays there as long as the pathogen is present. When the pathogen is no longer a threat, the phagocytes stop producing the pyrogen and the body’s set point for temperature returns to normal. When this happens, the body needs to lose the excess heat, which it does through sweating and dilating the blood vessels in the skin. The result is that the skin is warm and flushed.
- Specific immunity is directed against a specific pathogen. It uses two mechanisms: cellular immunity and humoral immunity. Cellular (cell-mediated) immunity aims to destroy foreign cells or host cells that have become infected with a pathogen. Humoral (antibody-mediated) immunity focuses on pathogens outside the host cells; it sends out antibodies to “mark” a pathogen for later destruction. Both system use lymphocytes and antibodies.
- Lymphocytes fall into one of three classes: natural killer cells, T lymphocytes, and B lymphocytes. T lymphocytes (or T cells) develop from stem cells in red bone marrow. Before T cells fully mature, they leave the bone marrow and travel to the thymus gland, where they remain until fully functional. Once T cells are immunocompetent—that is, capable of recognizing antigens—they leave the thymus and migrate to lymphatic organs and tissues throughout the body. B lymphocytes (or B cells) also begin life as stem cells in red bone marrow. Unlike T cells, B cells remain in bone marrow until they are fully mature. Once mature, B cells leave the bone marrow for lymphatic organs and tissues.
- Antibodies [also known as immunoglobulins (Ig)] are gamma-globulin proteins formed by B cells; they’re found in plasma and body secretions. Antibodies consist of chains of protein joined in a way that resembles a capital letter “Y” or “T.” The end of each arm of the Y is uniquely shaped, allowing each antibody to combine with a specific antigen. An antigen is any molecule that triggers an immune response. Any foreign substance is said to be antigenic.
- Active immunity is permanent (or at least long lasting). Passive immunity lasts only a few months because the body doesn’t develop a memory for the pathogen.
- Natural active immunity occurs when the body produces antibodies or T cells after being exposed to a particular antigen. For example, if you become ill with the measles, your body will produce antibodies to this particular virus, making you immune to infection in the future. Artificial active immunity results when the body makes T cells and antibodies against a disease as a result of a vaccination (such as for tetanus or influenza). By injecting a vaccine containing dead or weakened (attenuated) pathogens, the recipient’s body produces an immune response without actually developing the illness. Natural passive immunity results when a fetus acquires antibodies from the mother through the placenta, or when a baby acquires them through breast-feeding. Artificial passive immunity involves obtaining serum from a person or animal that has produced antibodies against a certain pathogen and then injecting it into someone else. This is typically used in emergencies for treatment of rabies and botulism.
- Cytotoxic T cells carry out the attack; also called killer T cells but are not to be confused with natural killer cells. Helper T cells play a supportive role. Memory T cells remember the pathogen in case of future infection.
- The immune process begins when a phagocyte (such as a macrophage, reticular cell, or B cell) ingests an antigen. The phagocyte, called an antigen-presenting cell (APC), displays fragments of the antigen on its surface—a process called antigen presentation; this alerts the immune system to the presence of a foreign antigen. When a T cell spots the foreign antigen, it binds to it. This activates (or sensitizes) the T cell; the T cell divides repeatedly to form clones—identical T cells already sensitized to the antigen. Some of these T cells become effector cells (such as cytotoxic T cells and helper T cells), which will carry out the attack, whereas others become memory T cells. The cytotoxic T cell binds to the surface of the antigen and delivers a toxic dose of chemicals that will kill it. Helper T cells support the attack by secreting the chemical interleukins, which attracts neutrophils, natural killer cells, and macrophages. It also stimulates the production of T and B cells.
- Humoral immunity does not destroy the antigen directly; instead, it uses antibodies to mark it for later destruction.
- The surface of a B cell contains thousands of receptors for a specific antigen. When the antigen specific to that receptor comes along, it binds to the B cell. The B cell engulfs the antigen and displays some of the antigen’s fragments on its surface. A helper T cell binds to the presented antigen and secretes interleukins, which activates the B cell. The B cell begins to rapidly reproduce, creating a clone, or family, of identical B cells programmed against the same antigen. Some of the cloned B cells become effector B cells or memory B cells; most become plasma cells. The plasma cells secrete large numbers of antibodies. Antibodies stop the antigens through a number of different means, including: Binding to the antigen’s attachment points, preventing it from attaching to a human cell Triggering agglutination (as in the antigen–antibody reaction), which contains the antigen and makes it easier for phagocytes to do their work Promoting the binding of complement proteins to the invading cell, thus setting off the complement cascade, which ends with the destruction of the invading microorganism
- When someone with a genetic predisposition to an allergy (such as ragweed) is first exposed to the allergen, the body produces large amounts of the antibody IgE specific to ragweed. These antibodies bind to mast cells. Although this response doesn’t produce an allergic reaction, the person is now sensitized to ragweed. When the person encounters ragweed at a later date, the allergen binds to the antibodies already in the body. If the allergen links two or more antibodies, the mast cells release histamine and other inflammatory chemicals. Histamine causes inflammatory responses that produce the symptoms of an allergy, such as runny nose, watery eyes, congestion, and hives. A severe, immediate allergic reaction that affects the whole body is anaphylaxis. Anaphylactic shock occurs when symptoms worsen to the point of circulatory shock; sudden death can occur.