chapter 9

Question 1

Disease-causing microorganism, such as a bacterium or virus.

Answer 1

Pathogen

Question 2

A complex group of cells, proteins, and structures of the lymphatic system that work together to provide the immune response

Answer 2

Immune system

Question 3

Differentiate between:

● Barriers to entry or ways of expelling or neutralizing pathogens before they can do harm. These include skin, stomach acid, tears, and such actions as vomiting and defecation.

● Nonspecific defense mechanisms. Nonspecific defenses help the body respond to generalized tissue damage and many of the more common or obvious pathogens, including most bacteria and some viruses.

● Specific defense mechanisms. These
enable the body to recognize and kill specific bacteria and other foreign cells and to neutralize viruses. Our specific defense mechanisms employ sophisticated weaponry indeed. The specific defense mechanisms are also the basis of immunity from future disease.

Answer 3

remember them

Question 4

Bacteria

Answer 4

single-celled living organisms Bacteria (singular: bacterium) are single-celled organisms that do not have a nucleus or membrane-bound organelles. All the DNA in most bacteria is contained in just one chromosome, which usually forms a continuous loop that is anchored to the plasma membrane. Bacterial ribosomes are smaller than ours and float freely in the cytoplasm. The outer surface of bacteria is covered by a rigid cell wall that gives bacteria their distinctive shapes, including spheres, rods, and spirals (Figure 9.1). Judging by their variety and numbers, bacteria are among the most successful organisms on Earth. Although they are smaller than the typical human cell, their small size is actually an advantage. Like all living organisms, bacteria need energy and raw materials to maintain life and to grow and divide. Their small size means that bacteria have a high surface-to-volume ratio, a decided advantage when it comes to diffusion, the means by which they obtain raw materials and get rid of wastes. Like our own cells, bacteria use ATP as a direct energy source and amino acids for making proteins. They store energy as carbohydrates and fats. Where do they obtain those raw materials? Anywhere they can. Some bacteria break down raw sewage and cause the decomposition of dead animals and plants, thereby playing an essential role in the recycling of energy and raw materials. Others obtain nutrients from the soil and air. Humans have learned to harness bacteria to produce commercial products, including antibiotic drugs, hormones, vaccines, and foods ranging from sauerkraut to soy sauce. Some bacteria even live within our digestive tract, drawing energy from the food we eat in exchange for manufacturing vitamins or controlling the populations of other, more harmful bacteria. Life, as we know it, would not be possible without these little organisms. A few bacteria are pathogens, however. Pathogens rely on living human cells for their energy supply, and in the process they damage or kill the human cells. They cause pneumonia, tonsillitis, tuberculosis, botulism, toxic shock syndrome, syphilis, Lyme disease, and many other diseases. Although we concentrate on pathogens in this chapter, do keep in mind that most bacteria are harmless and many are even beneficial. Bacterial infections are generally treated with antibiotics—chemotherapeutic agents that inhibit or abolish the growth of bacteria, fungi, and protozoa.

Question 5

Viruses

Answer 5

tiny infectious agents Viruses are extremely small infectious agents, perhaps onehundredth the size of a bacterium and one-thousandth the size of a typical eukaryotic cell (Figure 9.2). Structurally, a virus is very simple, consisting solely of a small piece of genetic material (either RNA or DNA) surrounded by a protein coat. Viruses have no organelles of their own, so they can’t grow and reproduce without access to the organelles of eukaryotic cells. Are viruses alive? Biologists are divided on the answer to this question. Most would say that viruses are not alive because they cannot reproduce on their own. Viruses have no observable activity associated with life when they are not in contact with another living cell. However, when they enter a living cell, they take it over and use the cell’s organelles to replicate. Viruses have several ways of gaining entry into living cells. Most viruses that infect human cells are taken into the cell cytoplasm by endocytosis; once inside the cell, the protein coats are dissolved and the viral genetic material is released for incorporation into the cell’s genetic material. Other viruses merge their outer coat with the cell membrane and release their genetic contents into the cell’s cytoplasm. Still other viruses attach to the outer surface of the cell membrane and inject just their genetic material into the cell, much as a needle and syringe inject drugs into the body. Regardless of the method of entry, the presence of the viral genetic material causes the cell to begin producing thousands of copies of the virus instead of carrying out its own metabolic activities. Sometimes the newly formed viruses are released by a type of budding from the cell membrane while the cell is still alive. In other cases, the cell becomes so packed with viruses that it dies and bursts, releasing a huge number of viruses all at once. Diseases caused by different types of viruses range from serious—AIDS, hepatitis, encephalitis, rabies—to annoying—colds, warts, or chicken pox. Viral infections can be minor for some people but serious for others. An otherwise healthy person may be ill for only a few days with a viral infection, whereas someone who is very young, very old, or in poor health may die. Antibiotics generally don’t work against viral infections. The best ways to cure a viral infection are either to prevent the viruses from entering living cells or to stop an infected host cell from producing more viruses.

Question 6

Prions

Answer 6

: infectious proteins In 1986, scientists identified a disease in British cattle that destroyed nerve cells in the animals’ brains and spinal cords, causing the animals to stagger, jerk, tremble, and exhibit other bizarre behaviors. The press nicknamed the condition “mad cow disease.” Then between 1994 and 1995, 10 Britons aged 19 to 39 developed signs of a new human disease called variant Creutzfeldt-Jakob disease (vCJD). Eight of them died. Alarmingly, researchers found that all of the vCJD patients had eaten beef from animals suspected of having mad cow disease. In 1996, scientists confirmed that a prion was responsible for both the mad cow disease and the first 10 cases of vCJD. A prion is a misfolded form of a normal brain cell protein. But it is not just a misfolded protein—it is misfolded protein that can trigger the misfolding of nearby normal forms of the protein as well. Once prions enter a nerve cell, the misfolding process becomes self-propagating—one prion produces another, which produces another, and so on. Eventually, so many prions accumulate within infected brain cells that the cells die and burst, releasing prions to infect other brain cells. The death of nerve cells accounts for the debilitating neurological symptoms and progressive degeneration seen in both mad cow disease and human vCJD. Prions are resistant to cooking, freezing, and even drying. There is no known cure for prion infection. Because infection occurs when humans (or cattle) eat prion-infected cattle tissues, the best way to prevent vCJD in humans is to limit the spread of mad cow disease in cattle. Global cooperation is making this possible. In 1994, the European Union banned the use of mammalian meat and bone meal products as cattle feed, and since that time the number of cases of mad cow disease has fallen dramatically.

Question 7

Which of these are antibiotics effective against?

Answer 7

bacteria

Question 8

Summarize the three functions of the lymphatic system

Answer 8

● It helps maintain the volume of blood in the cardiovascular system.

● It transports fats and fat-soluble vitamins absorbed from the digestive system to the cardiovascular system.

● It defends the body against infection.

Question 9

a milky body fluid that contains white blood cells, proteins, fats, and the occasional bacterium and virus.

Answer 9

lymph

Question 10

What is the function of lymph nodes?

Answer 10

Lymph nodes remove microorganisms, cellular debris, and abnormal cells from the lymph before returning it to the cardiovascular system. There are hundreds of lymph nodes, clustered in the areas of the digestive tract, neck, armpits, and groin (Figure 9.4). They vary in diameter from about 1 millimeter to 2.5 centimeters. Each node is enclosed in a dense capsule of connective tissue pierced by lymphatic vessels. Inside each node are connective tissue and two types of white blood cells, macrophages and lymphocytes, which identify microorganisms and remove them. (Macrophages and lymphocytes are discussed in greater detail in later sections.) The lymphatic vessels carry lymph into and out of each node (see Figure 9.3). Valves within these vessels ensure that lymph flows only in one direction. As the fluid flows through a node, the macrophages destroy foreign cells by phagocytosis, and the lymphocytes activate other defense mechanisms. The cleansed lymph fluid flows out of the node and continues on its path to the veins.

Question 11

What is the function of the spleen?

Answer 11

The largest lymphatic organ, the spleen, is a soft, fistsized mass located in the upper-left abdominal cavity. The spleen is covered with a dense capsule of connective tissue interspersed with smooth muscle cells. Inside the organ are two types of tissue, called red pulp and white pulp. The spleen has two main functions: it controls the quality of circulating red blood cells by removing the old and damaged ones, and it helps fight infection. The red pulp contains macrophages that scavenge and break down microorganisms as well as old and damaged red blood cells and platelets. The cleansed blood is then stored in the red pulp. Your body can call on this reserve for extra blood in case of blood loss or a fall in blood pressure, or whenever you need extra oxygen-carrying capacity. The white pulp contains primarily lymphocytes searching for foreign pathogens; it does not store blood. Notice that the main distinction between the spleen and lymph nodes is which fluid they cleanse—the spleen cleanses the blood, and the lymph nodes cleanse lymph. Together, they keep the circulating body fluids relatively free of damaged cells and microorganisms. A number of diseases, such as infectious mononucleosis and leukemia, cause the spleen to enlarge. The swollen spleen can sometimes be felt as a lump in the upper-left abdomen. A strong blow to the abdomen can rupture the spleen, causing severe internal bleeding. In this case, surgical removal of the spleen may be necessary to forestall a fatal hemorrhage. We can live without a spleen because its functions are shared by the lymph glands, liver, and red bone marrow. However, people who have had their spleen removed are often a little more vulnerable to infections.

Question 12

Where is the thymus gland located? What types of cells mature in the thymus?
Are these cells important in specific or nonspecific defenses? Yes

Answer 12

The thymus gland is located in the lower neck, behind the sternum and just above the heart. Encased in connective tissue, the gland contains lymphocytes and epithelial cells. The thymus gland secretes two hormones, thymosin and thymopoietin, that cause certain lymphocytes called T lymphocytes (T cells) to mature and take an active role in specific defenses. The size and activity level of the thymus gland vary with age. It is largest and most active during childhood. During adolescence, it stops growing and then slowly starts to shrink. By that time, our defense mechanisms are typically well established. In old age, the thymus gland may disappear entirely, to be replaced by fibrous and fatty tissue.

Question 13

Where are tonsils located? What is the function of tonsils?

Answer 13

tonsils are masses of lymphatic tissue near the entrance to the throat. Lymphocytes in the tonsils gather and filter out many of the microorganisms that enter the throat in food or air. We actually have several tonsils, and some are not readily visible. The familiar tonsils at the back of the throat are the largest and most often infected. When they become infected, the resulting inflammation is called tonsillitis. If the infection becomes serious, the tissues can be surgically removed. Lymphatic tissue called the adenoids lies at the back of the nasal passages. The adenoids tend to enlarge during early childhood, but in most people, they start to shrink after age 5 and usually disappear by puberty. In some cases, they continue to enlarge and obstruct airflow from nose to throat. This can cause mouth breathing, a nasal voice, and snoring. Like the tonsils, the adenoids can be surgically removed if they grow large enough to cause problems.

Question 14

Summarize how skin serves to keep pathogens out of the body.

Answer 14

The most important barrier against entry of any pathogen into our bodies is the skin. Skin has four key attributes that make it such an effective barrier:
(1) its structure,

(2) the fact that it is constantly being replaced,
(3) its acidic pH, and
(4) the production of an antibiotic by sweat glands. Regarding structure, the outermost layers of the skin’s epidermis consist of dead, dried-out epithelial cells. These cells contain a fibrous protein called keratin, which is also a primary component of fingernails and hair. When skin cells die and their water content evaporates, the keratin forms a dry, tough, somewhat elastic barrier to entry by microorganisms. Skin is continually being renewed throughout life. Dead cells shed from the surface are replaced by new cells at the base of the epidermis. Any pathogens deposited on the surface are shed along with the dead cells. Healthy skin has a pH of about 5 to 6, primarily because of the sweat produced by sweat glands. This relatively low (acidic) pH makes skin a hostile environment for many microorganisms. Sweat glands produce and secrete dermicidin, a natural antimicrobial peptide. Dermicidin is effective against a range of harmful bacteria, as well as some fungi. For further proof of intact skin’s effectiveness as a barrier to infection, look at what happens when the skin is damaged by a cut or scratch. If the damage reaches the moist layers of living cells underneath the skin, you may see signs of infection in the area within a few days. One of the most critical problems in treating patients with extensive burns is the infections that often result from the loss of the barrier function of skin.

Question 15

What is the name of the protective protein found in the outer layer of skin (and hair and nails)?

Answer 15

keratin

Question 16

Is skin basic, neutral, or acidic in terms of pH?

Answer 16

acidic

Question 17

What is the role of sweat glands in pathogen defense?

Answer 17

Sweat glands produce and secrete dermicidin, a natural antimicrobial peptide. Dermicidin is effective against a range of harmful bacteria, as well as some fungi.

Question 18

Summarize how the following keep pathogens from infecting us: Tears, saliva, earwax

Answer 18

Tears, saliva, and earwax Although we may not think of tears as a defense mechanism, they perform a valuable service by lubricating the eyes and washing away particles. Tears and saliva both contain lysozyme, an enzyme that kills many bacteria. In addition, saliva lubricates the delicate tissues inside the mouth so that they do not dry out and crack. It also rinses microorganisms safely from the mouth into the stomach, where most of them are killed by stomach acid. Earwax traps small particles and microorganisms.

Question 19

Summarize how the following keep pathogens from infecting us: Mucus

Answer 19

Mucus is a thick, gel-like material secreted by cells at various surfaces of the body, including the lining of the digestive tract and the branching airways of the respiratory system. Microorganisms that come into contact with the sticky mucus become mired and cannot gain access to the cells beneath. In addition, the cells of the airways have tiny hairlike projections called cilia that beat constantly in a wavelike motion to sweep mucus upward into the throat. There we get rid of the mucus by coughing or swallowing it. Sometimes, we remove mucus and microorganisms by sneezing, which is also one of the primary ways we pass microorganisms to other people (Figure 9.5).

Question 20

Summarize how the following keep pathogens from infecting us: Digestive and vaginal acids

Answer 20

Undiluted digestive acid is strong enough to kill nearly all pathogens that enter the digestive tract on an empty stomach. Only one strain of bacteria, Helicobacter pylori, has actually evolved to thrive in the highly acidic environment of the stomach. H. pylori is now known to contribute to many cases of stomach ulcers (see Chapter 14). Vaginal secretions are slightly acidic, too, though not nearly as acidic as stomach secretions.

Question 21

Summarize how the following keep pathogens from infecting us: Vomiting, urination, and defecation

Answer 21

Vomiting, though unpleasant, is certainly an effective way of ridding the body of toxic or infected stomach contents. Generally speaking, the urinary system does not have a resident population of bacteria. Urine is usually slightly acidic, and in addition the constant flushing action of urination tends to keep bacterial populations low. Urine pH can vary from fairly acidic to slightly basic, depending on diet. Some physicians advise patients with bladder or urethral infections to drink cranberry juice, which is acidic. The increased acidity of the urine inhibits bacterial growth, and the increased urine volume flushes the bacteria out. The movement of feces and the act of defecation also help remove microorganisms from the digestive tract. When we become ill, the muscles in the intestinal wall may start to contract more vigorously, and the intestine may secrete additional fluid into the feces. The result is diarrhea—increased fluidity, frequency, or volume of bowel movements. Unpleasant though diarrhea may be, mild cases serve a useful function by speeding the removal of pathogens.

Question 22

Summarize how the following keep pathogens from infecting us:Resident bacteria

Answer 22

Certain strains of beneficial bacteria normally live in the mucous membranes lining the vagina and the digestive tract. They help control population levels of more harmful organisms by competing successfully against them for food. They may also make the body less vulnerable to pathogens. For example, Lactobacillus bacteria in the vagina produce a substance that lowers vaginal pH to levels that many fungi and bacteria cannot tolerate. One might ask how any beneficial bacteria ever get to the small and large intestine if they have to pass through the stomach first. The answer is that following a meal the stomach contents are not so acidic because food both dilutes and buffers the stomach acid, so some bacteria pass through the stomach with the food we eat.

Question 23

Would it be a good idea to take enough antibotics to kill all the bacteria in our bodies?

Answer 23

no

Question 24

The complement system, or complement, comprises at least 20 plasma proteins that circulate in the blood and complement, or assist, other defense mechanisms. Normally, these proteins circulate in an inactive state. When activated by the presence of an infection, however, they become a potent defense force. Once one protein is activated, it activates another, leading to a cascade of reactions. Each protein in the complement system can activate many others, creating a powerful “domino effect.” Figure 9.6 shows how some activated complement proteins attack and destroy bacteria. 1 Activated complement proteins link together, forming protein complexes that create large holes through the bacterial cell wall. 2 Water and salts leak into the bacterium through the holes. 3 Eventually, the bacterium swells and bursts (lyses).

Answer 24

Complement system

Question 25

As noted in the chapter on blood, phagocytes are white blood cells that destroy foreign cells through the process of phagocytosis (Figure 9.7). 1 A phagocyte first captures a bacterium with its cytoplasmic extensions. 2 Then it draws the bacterium in, eventually engulfing it (endocytosis) and 3 enclosing it in a membrane-bound vesicle. 4 Inside the cell, the vesicle containing the bacterium fuses with lysosomes, and 5 the powerful enzymes in the lysosomes dissolve the bacterial membranes. Once digestion is complete, 6 the phagocyte jettisons the bacterial wastes by exocytosis.

Answer 25

Phagocytes

Question 26

Any type of tissue injury—whether infection, burns, irritating chemicals, or physical trauma—triggers a series of related events collectively called the inflammatory response, or inflammation. Inflammation has four outward signs: redness, warmth, swelling, and pain. Although these may not sound like positive developments, the events that cause these signs prevent the damage from spreading, dispose of cellular debris and pathogens, and set the stage for tissue-repair mechanisms. The inflammatory response starts whenever tissues are injured (Figure 9.8). The release of chemicals from damaged cells sounds the alarm for the process to begin. 1 These chemicals stimulate mast cells, which are connective tissue cells specialized to release histamine. Histamine promotes vasodilation of neighboring small blood vessels. White blood cells called basophils also secrete histamine. 2 Chemicals released by the tissue damage also activate complement proteins, which diffuse out of the leaky capillaries and begin destroying bacteria. Recall that most white blood cells (phagocytes) are too large to cross capillary walls. 3 As histamine dilates blood vessels, however, the endothelial cells in vessel walls pull slightly apart, and the vessels become more permeable. This allows additional phagocytes to squeeze through capillary walls into the interstitial fluid. There they attack foreign organisms and damaged cells. After destroying pathogens, some phagocytes travel to the lymphatic system, where their presence activates lymphocytes to initiate specific defense mechanisms (discussed later). Vasodilation brings more blood into the injured area, making it red and warm. The rising temperature increases phagocyte activity. The increased leakiness of capillary walls allows more fluid to seep into tissue spaces, causing swelling. The extra fluid dilutes pathogens and toxins and brings in clotting proteins that form a fibrin mesh to wall off the damaged area from healthy tissue. As a bonus, the fluid carries in extra oxygen and nutrients to promote tissue healing and carries away dead cells, microorganisms, and other debris from the area. Swollen tissues press against nearby nerve endings. This swelling, plus the sensitizing effects of inflammatory chemicals, creates the sensation of pain that accompanies inflammation. However, even pain can be positive. The discomfort hinders active movement and forces the injured person to rest, facilitating the healing process.

Answer 26

Inflammatory response

Question 27

Natural killer (NK) cells are a group of white blood cells (lymphocytes) that destroy tumor cells and cells infected by viruses. NK cells are able to recognize certain changes that take place in the plasma membranes of tumor cells and virus-infected cells. The name natural killer reflects the fact that NK cells are nonspecific killers, unlike other killer cells discussed later in this chapter that target only specific enemies. NK cells are not phagocytes. Instead, they release chemicals that break down their targets’ cell membranes. Soon after an NK attack, the target cell’s membrane develops holes, and its nucleus rapidly disintegrates. NK cells also secrete substances that enhance the inflammatory response.

Answer 27

Natural killer

Question 28

One of the most interesting defense mechanisms is an early warning system between virus-infected and still-healthy cells. As mentioned earlier, viruses cannot reproduce on their own. Instead, they invade body cells and use the cells’ machinery to make more viruses. Cells that become infected by viruses secrete a group of proteins called interferons. Interferons diffuse to nearby healthy cells, bind to their cell membranes, and stimulate the healthy cells to produce proteins that interfere with the synthesis of viral proteins, making it harder for the viruses to infect the protected cells. Interferons are now being produced in pharmaceutical laboratories. At least one interferon protein (alpha interferon) has shown promise against certain viral diseases, including genital warts, hepatitis B, and one form of leukemia.

Answer 28

Interferons

Question 29

A final weapon in our second line of defense is fever, an abnormally high body temperature. Your body’s “thermostat” is set to approximately 98.6°F (37°C), with a normal range of about 97–99°F (36–37.2°C). When macrophages detect and attack bacteria, viruses, or other foreign substances, they release certain chemicals into the bloodstream. These chemicals, called pyrogens, cause the brain to reset your thermostat to a higher temperature. There is a tendency to treat all fevers as if they were a problem. But a modest fever may be beneficial because it makes our internal environment less hospitable to pathogens and enhances the body’s ability to fight infections. Fever increases the metabolic rate of body cells, speeding up both defense mechanisms and tissue-repair processes. When the infection is gone, the process reverses: Macrophages stop releasing pyrogens, the thermostat setting returns to normal, and your fever “breaks.” Although a moderate fever tends to be beneficial, a high fever can be dangerous. Because the chemical bonds that give a protein its shape are relatively weak, they may be broken by high temperatures. If that happens, the shape of the protein will change and the protein may not function normally. It’s a good idea to monitor the course of any fever, particularly in children and older adults. Health professionals recommend seeking medical advice for any fever that lasts longer than two days or rises above 100°F.

Answer 29

Fever

Question 30

inflammation. Inflammation has four outward signs: redness, warmth, swelling, and pain. Although these may not sound like positive developments, the events that cause these signs prevent the damage from spreading, dispose of cellular debris and pathogens, and set the stage for tissue-repair mechanisms.

Answer 30

inflammation

Question 31

How does histamine help phagocytes fight pathogens?

Answer 31

Histamine promotes vasodilation of neighboring small blood vessels. White blood cells called basophils also secrete histamine.

Question 32

Is a mild fever a good or bad thing if you are fighting an infection?

Answer 32

Yes if it is mild it can fight

Question 33

Summarize the three major characteristics of the immune response.

Answer 33

The immune response has three important characteristics: ● It recognizes and targets specific pathogens or foreign substances. ● It has a “memory,” the capability to store information from past exposures to a pathogen so that it can respond more quickly to later invasions by the same pathogen. ● It protects the entire body; the resulting immunity is not limited to the site of infection.

Question 34

Define antigen

Answer 34

An antigen is any substance that mobilizes the immune system and provokes an immune response. Generally, antigens are large protein or polysaccharide molecules. In much the same way that a key fits a lock, each antigen has a unique shape, and every bacterium or virus has a different one. The immune system responds to each uniquely shaped antigen by producing specific antibodies to attack and inactivate the antigen (and the bacterium or virus carrying it).

Question 35

Differentiate between B cells and T cells

Answer 35

There are two types of these white blood cells: B lymphocytes and T lymphocytes, also called B cells and T cells. (Their names are based on where they mature: B cells mature in bone marrow; T cells in the thymus gland.) Although both types of lymphocytes can recognize and target antigen-bearing cells, they go about this task in different ways. B cells are responsible for antibody-mediated immunity. B cells produce antibodies—proteins that bind with and neutralize specific antigens. They release antibodies into the lymph, bloodstream, and tissue fluid, where they circulate throughout the body. Antibody-mediated immunity works best against viruses, bacteria, and foreign molecules that are soluble in blood and lymph. T cells are responsible for cell-mediated immunity, which depends on the actions of several types of T cells. Unlike B cells, T cells do not produce antibodies. Instead, some T cells directly attack foreign cells that carry antigens. Other T cells release proteins that help coordinate other aspects of the immune response, including the actions of T cells, B cells, and macrophages. Cell-mediated immunity protects us against parasites, bacteria, viruses, fungi, cancerous cells, and cells perceived as foreign (including, unfortunately, transplanted tissue—see section 9.8, Tissue rejection: a medical challenge). T cells can identify and kill infected human cells even before the cells have a chance to release new bacteria or viruses into the blood. Both B cells and T cells are activated by specific antigens, and both store information about their first exposure to a specific antigen in the form of memory cells. The presence of memory cells enables B cells and T cells to undergo clonal expansion (replication) more quickly the second and subsequent times they are exposed to the same antigen.

Question 36

Which of these cells turns into plasma cells that produce antibodies?

Answer 36

B cells

Question 37

Sequence the process of B cell activation

Answer 37

In adults, B cells mature in the bone marrow. As they mature, they develop unique surface receptors (with the same structure as an antibody) that allow them to recognize specific antigens (Figure 9.9). Then they travel in the bloodstream to the lymph nodes, spleen, and tonsils, where they remain inactive until they encounter a foreign cell with that particular antigen. 1 When a B cell with just the right surface receptor encounters the appropriate antigen, its surface receptors bind to the antigen. 2 This activates the B cell to grow and then multiply rapidly, producing more B cells exactly like the original and bearing the same surface receptors. The resulting identical cells, all descended from the same cell, are called clones. 3 Although the B cells themselves tend to remain in the lymphatic system, most of the cells of the clone are called plasma cells because they begin to secrete their antibodies into the lymph fluid and ultimately into the blood plasma. A typical plasma cell can make antibody molecules at a staggering rate—about 2,000 molecules per second. A plasma cell maintains this frantic pace for a few days and then dies, but its antibodies continue to circulate in blood and lymph. 4 Some of the clone cells become memory cells, longlived cells that remain inactive until that same antigen reappears in the body at some future date. Memory cells store information about the pathogen; if there is a second exposure, the immune response is even faster than the first time. Upon exposure, these memory cells quickly become plasma cells and start to secrete antibodies. Memory cells are the basis for long-term immunity.

Question 38

Summarize the process by which antibodies function to fight pathogens

Answer 38

) When the antibodies encounter matching antigens (Figure 9.10), 1 they bind to them and create an antigen antibody complex. Antibodies specialize in recognizing certain proteins; thus one particular antibody can bind to one particular antigen. 2 Some antibodies inactivate pathogens by causing the cells to agglutinate (clump together), preventing them from entering human cells and causing disease. 3 More commonly, the formation of an antigen antibody complex marks the antigen (and the foreign cell that carries it) for destruction either by phagocytes or by activated complement proteins.

Question 39

Which of these directly attacks and destroys other cells?

Answer 39

Cytotoxic t cells

Question 40

Differentiate between Helper T cells, Cytotoxic T cells, and Memory T cells

Answer 40

Helper T cells Produce cytokines. Enhance immune responses by stimulating other immune cells. Cytotoxic T cells Attack and destroy abnormal cells. Memory T cells Store information. Upon subsequent exposure to a specific antigen become helper and cytotoxic T cells.

Question 41

What does it actually mean to have 'immunity'?

Answer 41

When you are first exposed to an antigen, your immune system protects you with the wealth of defense mechanisms described so far. Your first exposure to a particular antigen generates a primary immune response. As we have seen, this involves recognition of the antigen and production and proliferation of B and T cells. Typically, the primary immune response has a lag time of three to six days after the antigen first appears. During this period, B cells specific to that antigen multiply and develop into plasma cells. Antibody concentrations rise, typically reaching their peak about 10–12 days after first exposure. Then they start to level off (Figure 9.16). However, as you have learned, B and T cells create a population of memory cells. The presence of these memory cells is the basis for immunity from disease. (The Latin word immunis means “safe” or “free from.”) Subsequent exposure to the pathogen elicits a secondary immune respons that is faster, longer lasting, and more effective than the first. Within hours after second exposure to an antigen, memory cells bind to the pathogen. New armies of T and plasma cells form, and within a few days antibody concentrations rise rapidly to much higher levels than in the primary response. Notice that antibody levels remain much higher in the body after second exposure. Memory cells are long-lived, and many retain their ability to generate a secondary immune response over a lifetime. The secondary immune response can be so effective that you don’t even realize you’ve been exposed to the pathogen a second time. At worst, you may experience only a fleeting sensation of feeling unwell. Some memory cells, such as the ones for the bacterial infection that causes tetanus, need to be reactivated (exposed to the pathogen again to undergo clonal expansion) every 10 years or so. Given this immunity, though, why is it possible to get a cold or the flu over and over, sometimes several times a year? One reason is that there are more than 100 different viruses that can cause colds and flu. Even if your latest respiratory ailment feels like the previous one, it may actually be due to an entirely different pathogen. Furthermore, the viruses that cause colds and flu evolve so rapidly that they are essentially different each year. Their antigens change enough that each one requires a different antibody, and each exposure triggers a primary response. Rapid evolution is their survival mechanism. Our survival mechanism is a healthy immune system.

Question 42

Can you be reinfected with the same virus twice?

Answer 42

yes

Question 43

What is the major difference in antibody response between the first and second exposures

Answer 43

the secondary exposure will be faster, stronger and longer

Question 44

How do vaccines function to produce immunity based on what you learned in section 9.6?

Answer 44

A vaccine works by training the immune system to recognize and combat pathogens, either viruses or bacteria. To do this, certain molecules from the pathogen must be introduced into the body to trigger an immune response. These molecules are called antigens, and they are present on all viruses and bacteria.

Question 45

Do vaccines cause the formation of plasma cells and antibodies against the antigen?

Answer 45

yes

Question 46

Do vaccines cause the formation of memory cells to fight off pathogens that have that antigen?

Answer 46

yes

Question 47

Differentiate between active and passive immunization? In which of these does the patient produce their own antibodies and produce memory cells for the future? In which of these is the patient given antibodies that are made by another organism and that only stay in circulation for a limited time?

Answer 47

1 active 2 passive

Question 48

What is it called when your immune system rejects and fights against a blood transfusion of the wrong blood type?

Answer 48

It’s called a transfusion reaction, and it can be fatal.

Question 49

What does this section say is the key to successful tissue or organ transplants? What is the risk of this however?

Answer 49

This phenomenon is called tissue rejection. A very common type of tissue rejection occurs with blood transfusion, when a donor’s blood is rejected by the recipient patient. It’s called a transfusion reaction, and it can be fatal. Surgical techniques for performing many organ transplants are really not that difficult. Historically, the major stumbling block to widespread transplantation of most organs has been the effectiveness of the immune system in rejecting foreign tissue. In the normal immune response, cytotoxic T cells swiftly attack and destroy any and all foreign cells. Before a transplant is even attempted, then, the donor’s and recipient’s ABO and other blood group antigens must first be determined. Next, donor and recipient tissues are tested to compare MHC antigens, because cytotoxic T cells target foreign MHC proteins. The closer the relationship between donor and recipient the better, because their MHC antigens are likely to be similar. Although successful transplants can be done between unrelated people, at least a 75% match between tissues is essential. After surgery, the patient must take immunosuppressive drugs that block the immune response, such as corticosteroid drugs to suppress inflammation or cytotoxic medications that kill rapidly dividing cells (to block activated lymphocytes). Immunosuppressive therapy can dramatically prolong the lives of transplant patients, but it brings other risks. An impaired immune system cannot protect the body as effectively against pathogens and abnormal cells, so patients who are taking immunosuppressive drugs are more vulnerable to infections and certain cancers. The key to a successful transplant is to suppress the immune system enough to prevent rejection, while preserving as much immune function as possible. Antibiotic drugs can help control infections as they arise. In recent years, three factors have made organ transplants a viable option for many people: (1) improvements in immunosuppressive drugs, (2) better techniques for cross-matching (or “typing”) tissue, and (3) national sharing of information and donor organs through organ-bank systems. The organ-bank system allows patients to receive the best matches possible regardless of where they live.

Question 50

Define: Allergy and allergen

Answer 50

An allergy is an inappropriate response of the immune system to an allergen (any substance that causes an allergic reaction). The key word is inappropriate: The allergen is not a dangerous pathogen, but the body reacts as if it were.

Question 51

Define: Autoimmune disorder

Answer 51

An autoimmune disorder occurs when the body's immune system attacks and destroys healthy body tissue by mistake. There are more than 80 types of autoimmune disorders.

Question 52

What is lupus erythematosus?

Answer 52

Lupus erythematosus (or lupus) is an autoimmune disorder in which the body attacks its own connective tissue. One type of lupus, called discoid lupus erythematosus, primarily affects areas of the skin exposed to sunlight. More serious is systemic lupus erythematosus, which may affect various tissues and organs including the heart, blood vessels, lungs, kidneys, joints, and brain. Lupus often starts as a red skin rash on the face or head. Other symptoms include fever, fatigue, joint pain, and weight loss. Spreading inflammation can lead to osteoarthritis (see Chapter 5), pericarditis (see Chapter 8), or pleurisy (inflammation of the lining of the lungs). Lupus affects nine times as many women as men. Typically, it occurs during childbearing age and is more common in certain racial groups such as African Americans, West Indians, and Chinese. Medications can reduce the inflammation and alleviate the symptoms

Question 53

What is rheumatoid arthritis?

Answer 53

Rheumatoid arthritis is a type of arthritis involving inflammation of the synovial membrane that lines certain joints (see Chapter 5). In rheumatoid arthritis, B cells produce antibodies against a protein in the cartilage of synovial membranes. The resulting immune response releases inflammatory chemicals that cause further tissue damage. At first, fingers, wrists, toes, or other joints become painful and stiff. Over time, the inflammation destroys joint cartilage and the neighboring bone. Eventually, bony tissue begins to break down and fuse, resulting in deformities (Figure 9.19) and reduced range of motion. The disease is intermittent, but with each recurrence the damage is progressively worse. Pain-relieving medications can help many people with rheumatoid arthritis, as can regular mild exercise and physical therapy to improve range of motion. Powerful drugs that neutralize chemicals in the inflammatory response can prevent joints from becoming deformed. Surgery to replace damaged joints with artificial joints can restore the ability to move and prevent painful disabilities.

Question 54

What is the relationship between AIDS and HIV? Over time what happens to the concentration of T cells in a person with HIV? Over time what happens to the concentration of HIV in the blood?

Answer 54

Immune deficiency is a general term for an immune system that is not functioning properly. One immune deficiency disease is severe combined immunodeficiency disease (SCID). For the rare person who inherits SCID, even a minor infection can become life-threatening. People with SCID have too few functional lymphocytes to defend the body against infections. The most common and best-known severe immune deficiency condition is AIDS (acquired immune deficiency syndrome). A syndrome is a medical term for a group of symptoms that occur together, and acquired means that a person catches a disease—in this case by becoming infected with HIV (human immunodeficiency virus). HIV targets helper T cells of the immune system Figure 9.20 shows the structure of HIV. The virus consists of nothing more than single-stranded RNA and enzymes, wrapped in two protein coats and a phospholipid membrane with protein spikes. It has no nucleus and no organelles. Like other viruses, HIV infects by entering a cell and using the cell’s machinery to reproduce. HIV targets helper T cells, gaining entry by attaching to CD4 receptors. HIV belongs to a particular class of viruses, called retroviruses, that have a unique way of replicating (Figure 9.21). Retroviruses first attach to the CD4 receptor of a helper T cell. The attachment fuses the retrovirus’s envelope with the cell’s membrane, releasing the viral RNA and enzymes into the cell. Under the influence of the viral enzymes and using the viral RNA as a template (a pattern), the host cell is forced to make a single strand of DNA complementary to the viral RNA, and from it a second strand of DNA complementary to the first. The new double-stranded DNA fragment is then inserted into the cell’s DNA. The cell, not recognizing the DNA as foreign, uses it to produce more viral RNA and proteins, which are then assembled into thousands of new viruses within the cell. The sheer magnitude of viral replication so saps the T cell’s energy that eventually it dies and ruptures, releasing the viral copies. The new viruses move on to infect other helper T cells. Notice that AIDS may not appear until years after initial HIV infection. Untreated AIDS is nearly always fatal. The AIDS epidemic: a global health issue AIDS was first described in 1981 when the Centers for Disease Control and Prevention in Atlanta began to notice a disturbing similarity between cases involving a strange collection of symptoms. (This illustrates the advantage of having a central clearinghouse for medical information.) It is now believed that HIV first infected humans in the 1960s in Africa after “jumping species” from other primates to humans. The worldwide damage done by HIV so far is truly astonishing. Today, more than 35 million people are living with HIV infection or AIDS, representing nearly 1% of the adult population worldwide. More than 5,700 people are newly diagnosed every day. So far, nearly 39 million people have died of AIDS. Sub-Saharan Africa has borne the brunt of the HIV epidemic. Today, 70% of all new cases of HIV infection occur in sub-Saharan Africa. Notably, the patterns of AIDS infection and transmission in Africa differ from those of industrialized countries. In the United States, more men than women are infected with HIV because the virus is generally transmitted via homosexual sex. In Africa, more than half of all HIV-infected persons are women who have contracted the virus heterosexually. Studies in several African nations have found that females aged 15–19 are four to five times more likely to be infected than males their age. According to one report, in Africa older HIV-infected men coerce or pay impoverished girls to have sex in the mistaken belief that sex with a virgin will cure AIDS. The problem of AIDS in sub-Saharan Africa is made worse by political, economic, and social instability. Some sub-Saharan African nations are at war or are suffering intense internal conflict. In many countries, the economies are weak, transportation is difficult, sanitation is poor, and there are too few hospitals and medical personnel. Only 37% of sub-Saharan Africans with HIV currently are being treated for their infections. And yet, there is hope on the horizon. Efforts to improve health care delivery by international agencies such as the World Health Organization (WHO) and various nongovernmental organizations such as Doctors Without Borders are beginning to have an effect. The number of new cases of HIV infection each year is now declining, even in sub-Saharan Africa. Worldwide, new HIV infections and AIDS-related deaths are down more than 35% from their peaks in the previous decade. The picture is even better in the United States, thanks to a well-developed AIDS reporting system and the availability of good health care. In the United States, the number of deaths has declined to under 14,000 per year after peaking sharply in 1995 at more than 50,000 per year (Figure 9.23). Most of the decline after 1995 is due to the availability of drugs that suppress the active component of an HIV infection and keep infected people alive much longer. However, to be completely effective the drugs need to be taken for the rest of the patient’s lifetime. Although the death rate from AIDS has stabilized in the United States, the number of persons living with HIV/ AIDS continues to climb. More than 500,000 persons are now living with AIDS in the United States. An additional 700,000 persons are currently infected with HIV but are not yet showing symptoms of AIDS. About 20% of all HIV-infected persons do not even know they are infected. Treatment of the rising number of persons living with HIV/ AIDS is likely to place a heavy financial burden on our health care system in the future. The current cost of treating an HIV-infected person with HIV suppressive drugs is about $14,000–$20,000 per year.