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Answers to Mastering Concepts Questions 29.1 1. List the cell types that participate in the body’s defenses, along with some of their functions. Phagocytes engulf and destroy bacteria; basophils and mast cells release chemicals to trigger inflammation and allergies; lymphocytes (T cells and B cells) coordinate defenses against specific pathogens. Natural killer cells attack cancerous and virus-infected cells. 2. What is the relationship between lymph and blood? Lymph is blood plasma that has leaked out of capillaries and is traveling in the lymphatic system, eventually to be cleaned, filtered, and returned to the bloodstream. 3. What are the functions of the main lymphoid organs? The main lymphoid organs are the thymus, spleen, lymph nodes, adenoids, and tonsils. All of these lymphoid organs have a similar function: they are sites with high concentrations of lymphocytes and macrophages. In lymphoid organs, lymph is cleaned and filtered, and bacteria and tissue debris are removed. 4. What are the two subdivisions of the immune system? The immune system is divided into innate defenses and adaptive immunity. 29.2 1. What are innate defenses? Innate defenses are always in place and protect against all kinds of pathogens. Barriers, white blood cells, inflammation, the complement system, and fever are the innate defenses. 2. Describe the external barriers to infection. Barriers to infection in the human body include the skin, mucous membranes in the nose and throat, earwax, tears, cilia in the respiratory system, acid secretions of the stomach, and the normal bacteria in the gut and elsewhere. 3. Which white blood cells participate in innate defenses? Phagocytes destroy invaders by engulfing bacteria, triggering inflammation, or secreting chemicals that destroy pathogens. 4. What are some examples of antimicrobial biochemicals? Examples of antimicrobial biochemicals include complement proteins that destroy pathogens, attract phagocytes, or cause mast cells to release histamine. 5. How is fever protective? Fever creates an environment that is hostile to bacteria and viruses. Not only is the warmer temperature lethal to some bacteria and viruses, it also reduces the iron level in the blood, depriving bacteria and fungi of iron and reducing their replication. Fever also increases the activity level of phagocytes. 6. Describe the events of inflammation. Basophils and mast cells secrete histamine, causing blood vessel dilation and increased permeability. The phagocytic white blood cells leave the blood and enter the tissue to destroy bacteria and damaged cells. 29.3 1. What is the relationship between antigens and antibodies? Antigens are molecules that trigger the production of antibodies. 2. What is the role of macrophages in adaptive immunity? In adaptive immunity, the macrophages engulf invading cells and then act as “teachers” that display foreign antigens on their surfaces. Helper T cells recognize these antigens and bind to the macrophage. This initiates other phases of the cellular response and humoral response. 3. What are the two subdivisions of adaptive immunity, and which cell types participate in each? The two subdivisions of adaptive immunity are the cellular response and the humoral response. Helper T cells, memory T cells, and cytotoxic T cells participate in the cellular response. Memory B cells and plasma cells participate in the humoral response. 4. What do cytotoxic T cells do? Cytotoxic T cells engulf and destroy cancerous cells or cells that are infected with viruses. 5. Describe the structure and function of an antibody. An antibody is a Y-shaped protein that matches a specific antigen. Upon encountering an invader with a matching antigen, the antibody binds to the antigen. This may make the antigen more noticeable to macrophages, may inactivate a microbe, or may neutralize its toxins. Viruses that are coated with antibodies may not be able to connect with their target cells. Antibodies also trigger the production of complement proteins that destroy microbes. 6. What happens after a B cell is stimulated? A stimulated B cell can proliferate, producing memory cells, or it can produce plasma cells that make antibodies. 7. Explain the difference between the primary and secondary immune responses. The primary immune response is triggered the first time the body encounters a pathogen; a secondary immune response is triggered on subsequent exposure. A primary immune response may take days or weeks to respond to the infection. The secondary immune response is much quicker and stronger than the primary response. 8. What happens if an immune reaction persists after a pathogen is eliminated? It can lead to an autoimmune disorder where the immune system begins to attack the body’s own cells. 29.4 1. What is a vaccine? A vaccine is a mixture containing antigens that stimulate the primary immune response without actually causing disease. 2. List the main types of vaccine formulations. Vaccines can be composed of live but weakened pathogens, inactivated pathogens or toxins, subunits of pathogens, or recombinant component vaccines. 3. Why haven’t scientists been able to develop vaccines against HIV and the common cold? This viruses mutate too frequently for a vaccine to be effective. 29.5 1. What events might lead to autoimmunity? One event that might lead to autoimmunity includes the survival of lymphocytes that correspond to the body’s molecules. These ordinarily are destroyed early in development, but if they survive, they can cause autoimmunity. 2. How do HIV and SCID lead to immunodeficiency? HIV attacks helper T cells, causing the body’s immune response to fail. Once too many T cells are lost, the person becomes susceptible to opportunistic illnesses and infections. SCID is different from HIV infection in that SCID is not caused by a virus; instead, SCID children have a genetic lack of immune response. Their T cells and B cells do not function and they are prone to opportunistic infections. 3. Which cells and biochemicals participate in an allergic reaction? The cells that participate in an allergic reaction include B cells, plasma cells that secrete antibodies, and mast cells. Biochemicals include histamine released from mast cells. 29.6 1. How did researchers investigate the worm-allergy connection? The children were exposed to different allergens, including dust mites, on a small portion of their arms. The presence of welts indicated a positive allergic reaction. The students were also tested for the presence of parasitic worms. The researchers then compared the incidence of allergies among children with and without worms. 2. How do infectious worms lower the risk for allergies? Interleukin acts as a brake on the immune system, and it is released in response to the presence of parasitic worms. Therefore, children with worms produce interleukin and are less likely to suffer from allergies. Answers to Write It Out Questions 1. List and describe the components of the lymphatic system. The components of the lymphatic system are lymph (the colorless fluid that carries white blood cells; and that the lymphatic system transports, cleanses, and returns to the bloodstream); lymphoid organs such as the thymus, spleen, and lymph nodes (collections of lymphocytes embedded in loose connective tissue); and lymph vessels (tubes that absorb and carry lymph). 2. Explain the observation that lymphoid tissues are scattered in the skin, lungs, stomach, and intestines. The skin, lungs, stomach, and intestines are all in contact with the outside world, so they are places where pathogens are likely to enter the body. 3. How does the immune system interact with the circulatory system? Phagocytes, antibodies, and other immune system cells and substances originate in the immune system but travel in the bloodstream. In addition, lymph originates as blood plasma that leaks out of blood vessels. Once cleansed, lymph returns to the blood. 4. List the types of innate defenses. Innate defenses are always present and ready to function. They include physical barriers such as intact skin, mucus, earwax, and tears; white blood cells; inflammation; antimicrobial biochemicals; and fever. 5. Dead phagocytes are one component of pus. Why is pus a sure sign of infection? Phagocytes fight infection by engulfing and destroying bacteria and damaged tissues. Pus is a fluid containing these white blood cells and other debris. 6. How can inflammation be both helpful and harmful? Inflammation can be helpful because it recruits immune components, helps clear debris, and creates an environment hostile to microorganisms around the site of injury or infection. It can be harmful, as it can cause the site to become swollen and painful. Also, joints and other body parts can become inflamed even if they are uninfected, causing great discomfort. 7. If you take an antiinflammatory drug after spraining your ankle, what symptoms should be reduced? The drug will keep the vessels from dilating and becoming more permeable; redness, swelling, and heat should therefore be reduced. The drug should also relieve pain because the lack of swelling in the tissues will keep pressure off surrounding nerves. 8. State the functions of antibodies, cytokines, and complement proteins. Antibodies are Y-shaped proteins that recognize and bind to specific antigens; cytokines are messenger chemicals that alter the activity of immune cells in many ways; complement proteins help destroy pathogens in the body. 9. What do a plasma cell and a memory cell descended from the same B cell have in common, and how do they differ? Both plasma and memory cells are formed after activation of B cells; plasma cells secrete a huge number of antibodies, whereas memory cells “remember” antigens the immune system has already encountered. 10. In your own words, write a paragraph describing the events of adaptive immunity, beginning with a macrophage engulfing a bacterial cell and ending with the production of memory cells. When a macrophage engulfs a bacterial cell, it displays the bacterial antigens on its surface. The antigen-presenting macrophage enters a lymph node, where the combination of self protein plus bacterial antigen attracts helper T cells. The helper T cells secrete chemicals that activate both B cells and T cells with receptors for the bacterial antigen. The activated B cells divide and differentiate into plasma cells that produce antibodies; some B cells also differentiate into memory cells. Likewise, activated T cells differentiate into memory cells and cytotoxic T cells that attack the bacterium. 11. How do the innate defenses and adaptive immunity cooperate to eradicate an infection? Innate defenses include nonspecific barriers, inflammation, chemical defenses, and fever. Phagocytosis is part of the body’s innate defense, but when macrophages display foreign antigens on their surfaces, they trigger the adaptive immune response. In adaptive immunity, immune cells produce antibodies and other defenses against specific antigens. 12. Which do you think would be more dangerous, a deficiency of T cells or a deficiency of B cells? Explain your reasoning. Both would be dangerous, but helper T cells coordinate the immune response (including triggering the activity of B cells), so a deficiency of T cells would be more dangerous. 13. One benefit of sexual reproduction is a genetically variable population. This genetic variation may help a population stay “one step ahead” of pathogen populations. Describe how genetic variability can enhance immunity. Ultimately, the immune response has a genetic basis. The more genetic variability in a population, the greater the potential to develop an immune response against a variety of pathogens. In addition, many pathogens attack only cells with specific receptor molecules. Genetic variability in the host population means more variants of those receptors, which makes it more difficult for the pathogen to constantly stay compatible. 14. What is a vaccine, and how is a vaccine different from an antibiotic? A vaccine is a substance that triggers a primary immune response, whereas an antibiotic is a drug that kills bacteria. A vaccine is different from an antibiotic because it does not destroy the pathogen. In addition, a vaccine triggers the production of memory cells that can quickly respond to a future infection. 15. Explain why it might be dangerous for a person with a weakened immune system to receive a vaccine consisting of a live, weakened pathogen. A live, weakened pathogen triggers a mild version of the illness. Most individuals would be able to handle such an infection, but an individual with a weakened immune system could become very ill. 16. Influenza viruses mutate rapidly, whereas the chickenpox virus does not. Why are people encouraged to receive vaccinations against influenza every year, whereas immunity to chickenpox lasts for decades? Vaccines work by “teaching” the recipient’s immune system to recognize a diseasecausing organism without actually causing the illness. Viruses have surface molecules that the immune system recognizes. If the virus frequently mutates, frequent vaccines are required to “teach” the immune system about the new variants. The memory of the slowto-mutate chickenpox virus lasts much longer. 17. If worm infections suppress the immune system as suggested in section 29.6, do you think vaccines should be more or less effective in children infected with worms? Explain your answer. Vaccines should be less effective in children with worms because vaccines trigger an active immune response, which would be suppressed. 18. What is an opportunistic infection? Explain the statement that opportunistic infections, not HIV alone, cause death in an end-stage AIDS patient. Opportunistic infections result from pathogens that the immune system can normally defeat. These pathogens can, however, cause infection in people with weakened immune systems. In end-stage AIDS patients, HIV has destroyed most helper T cells. The patient dies from opportunistic infections their immune system can no longer defeat. 19. How do SCID, AIDS, and allergies each relate to the function of the immune system? SCID is an inherited condition in which neither the T nor B cells function. AIDS is caused by HIV, which attacks and destroys helper T cells in the body. An allergy is an overreaction of the immune system to a harmless antigen. 20. What role do antibodies play in allergic reactions and in autoimmune disorders? In autoimmune disorders, antibodies attack the body’s own cells. In allergic reactions, antibodies bind to mast cells, causing them to release histamine and other allergy mediators. 21. How might a drug advertised as a “histamine blocker” relieve allergy symptoms? Histamine is an allergy mediator that causes blood vessels to dilate and triggers other harmful effects of an allergic reaction. Histamine blockers would stop this chain reaction. 22. Explain the difference between: clonal deletion and clonal selection; a natural killer cell and a cytotoxic T cell; antibodies and antigens; cell-mediated and humoral immunity; an autoimmune disorder and an immunodeficiency. Clonal deletion occurs during lymphocyte development, when lymphocytes that respond to antigens on the surface of the body’s cells are deleted; in clonal selection, a specific antigen stimulates a B cell to divide. Natural killer cells are part of innate immunity; cytotoxic T cells are part of adaptive immunity. Antibodies are Y-shaped proteins that recognize antigens, which are molecules that trigger the production of antigens. In cellmediated immunity, pathogens are destroyed in direct cell-to-cell contact; in humoral immunity, the main defense is antibodies. In an autoimmune disorder, the immune system attacks the body’s own cells; in an immunodeficiency, the immune system is lacking one or more essential components. 23. Search the Internet for evidence for and against the hypothesis that autism is associated with the thimerosal preservative that was once added to childhood vaccines (keep in mind that not all websites are equally credible). Do you find the evidence for or against the hypothesis most compelling? What sort of evidence would it take to change your position to the opposite side of the issue? [Answers will vary.] 24. Search the Internet for information about immune system disorders. Choose one illness to study in more detail. What are the characteristics of the disorder? Who is primarily affected? What causes the illness, and is there a treatment or cure? [Answers will vary.] 25. Search the Internet for ads for commercial products that claim to “boost the immune system.” Choose a product to investigate in detail. What specific claims do ads for the product make? What scientific evidence does the manufacturer offer in support of its claims? Based on what you know about the immune system, does the scientific evidence seem convincing? [Answers will vary.] Answers to Pull it Together Questions 1. How do innate defenses and adaptive immunity differ? Innate defenses protect against all pathogens and are in place throughout life; they do not change in their ability to protect you. Adaptive immunity is directed against specific pathogens. The adaptive immune system changes throughout life by “remembering” pathogens the body has already encountered. 2. What are examples of barriers that contribute to innate defenses? Any barricade that can slow or stop the movement of an invader qualifies as innate defense. Examples are intact skin, earwax, nose hairs, tears, mucus, and stomach acid. 3. Where do cytokines and complement proteins fit into this concept map? Cytokines and complement proteins are part of the innate defenses that are found when a pathogen gets past a barrier. 4. How do cytokines interact with the adaptive immune system? In general, cytokines are communication molecules that aid in the coordination of the immune response. Interleukins are one type of cytokine. Macrophages release interleukins that activate B and T cells. 5. How do lymph and lymph nodes fit into this concept map? "Macrophages" connect with the phrase "travel in" to "Lymph". "Lymph" then connects with the phrase "travels within lymph vessels to" to "Lymph nodes", which connects with the phrase "which contain" to "Helper T cells". 6. What are the roles of memory B cells and plasma B cells? When they are first produced, memory B cells do not respond to the invading antigen. Instead, they linger in the body, ready to launch a rapid response in case the same antigen is encountered again. Plasma B cells are active as soon as they form, releasing specific antibodies that neutralize an invading pathogen.