Mastering Concepts
34.1
1. List the cell types that are important in the body’s defenses, along with some of their
functions.
Neutrophils act as phagocytes, which engulf bacteria and debris; monocytes also develop
into macrophages. Eosinophils defend against worms and other multicellular parasites.
Basophils and mast cells release chemicals to trigger inflammation and allergies. Two
types of lymphocytes (T cells and B cells) coordinate defenses against specific
pathogens. Natural killer cells are lymphocytes that attack cancerous and virus-infected
cells.
2. List and describe the components of the lymphatic system.
Lymph is the colorless fluid that carries white blood cells and that the lymphatic system
transports, cleanses, and returns to the bloodstream. Lymphoid organs include the
thymus, spleen, and lymph nodes (collections of lymphocytes embedded in loose
connective tissue). Lymph vessels are the tubes that absorb and carry lymph.
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. What are the two subdivisions of the immune system?
The immune system is divided into innate defenses and adaptive immunity.
34.2
1. List five categories of innate defenses.
Barriers, white blood cells, inflammation, the complement system, and fever are the
innate defenses.
2. What are some barriers to infection in the human body?
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. How do white blood cells and macrophages destroy invaders?
Macrophages and neutrophils destroy invaders by phagocytosis. Other white blood cells
trigger inflammation or secrete chemicals that destroy pathogens.
4. 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. Inflammation can also be harmful because it can cause the site to become
swollen and painful. Also, joints and other body parts can become chronically inflamed
even if they are uninfected, causing great discomfort.
5. How is fever protective?
Fever creates an environment that is hostile to bacteria and viruses. Not only does the
warmer temperature inhibit 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.
34.3
1. What is the relationship between antigens and antibodies?
Antigens are molecules that trigger the production of antibodies.
2. 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.
3. In your own words, write a paragraph describing the events of adaptive immunity,
beginning with a pathogen entering a host’s body and ending with the production of
memory cells.
When a pathogen enters a host’s body, it may be present in body fluids and infect living
cells. A macrophage engulfs a pathogen in body fluids and displays the pathogen’s
antigens on its surface. The antigen-presenting macrophage enters a lymph node, where
the combination of self protein plus antigen attracts helper T cells. Activated helper T
cells divide into memory cells and effector cells, which secrete chemicals that activate B
cells and cytotoxic T cells with receptors for the antigen. The activated B cells divide
and differentiate into plasma cells that produce antibodies; some B cells also differentiate
into memory cells. Likewise, activated cytotoxic T cells differentiate into memory cells
and effector T cells that attack host cells already infected by the pathogen.
4. 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, inactivate a microbe, or neutralize its toxins.
Viruses that are coated with antibodies may not be able to attach to target cells.
Antibodies also trigger the production of complement proteins that destroy microbes.
5. What happens if an immune reaction persists after a pathogen is eliminated?
An immune system response that persists after a pathogen is eliminated may attack and
damage the body’s own tissues.
6. Explain the difference between the primary and secondary immune response.
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.
34.4
1. What is a vaccine?
A vaccine is a mixture containing antigens that stimulate the 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, or subunits of pathogens.
3. Why haven’t scientists been able to develop vaccines against HIV and the common
cold?
These viruses mutate too frequently for a vaccine to be effective.
34.5
1. What is autoimmunity?
In autoimmunity, the immune system attacks the body’s own molecules, potentially
leading to severe tissue and organ damage.
2. Which immune system cells does HIV attack, and what is the consequence?
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.
3. Which cells and biochemicals participate in an allergic reaction?
The cells that participate in an allergic reaction include B cells and mast cells.
Biochemicals include the allergen’s antigens, the antibodies secreted by plasma cells, and
the histamine released from mast cells.
4. How is the Rh factor important in determining whether a pregnant woman’s immune
system attacks her fetus?
Fetal cells can enter a woman’s bloodstream (usually during birth) and stimulate the
production of antibodies against the Rh antigen. An Rh-negative mother’s immune
system will form antibodies against the red blood cells of an Rh+ fetus. The first
pregnancy will be fine, but the second pregnancy of an Rh+ fetus in an Rh- mother will
result in the mother’s immune system destroying the red blood cells of her fetus.
34.6
1. How did researchers document the inverse relationship between worm infection and
allergies?
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 graphed the
incidence of allergies among children with and without worms.
2. Use figure 34.16 to describe the worm–allergy relationship.
Children with worms are less likely to suffer from allergies (exaggerated immune
responses) than are children without worms. The worms stimulate the production of an
interleukin that acts as a brake on the immune system.
Write It Out
1. 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.
2. Explain why a scraped knee increases the chance that pathogens will trigger an
adaptive immune response.
A scraped knee opens the skin, allowing pathogens to bypass an important innate
defense. Once in the body, the pathogens may multiply and initiate the adaptive immune
response.
3. Since fever has some protective effects, should we avoid taking fever-reducing
medications when ill? Use the Internet to research the consequences of overmedicating a
fever and the risk of allowing fever to rise too high.
Fever is the immune system’s natural response to invading pathogens. It creates a harsh
environment for many invaders, helping the immune system to eliminate them. Taking
medications to reduce a moderate to high fever (less than 105 degrees F) is generally not
recommended since it can prolong the illness. However, very high fevers can sometimes
cause seizures in children. A high fever can also cause dehydration, so a person with a
fever should drink plenty of water.
4. 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.
5. In an effort to reduce allergic responses, a drug company wishes to develop a
medication that binds to and kills mast cells. What would be a side effect of this drug?
Mast cells release the histamine that accompanies an allergic reaction. However, mast
cells and histamine also have important functions in innate immunity against pathogens.
When pathogens enter the skin, mast cells release histamine, which triggers
inflammation. These immune reactions are helpful because they recruit other immune
components, help clear debris, and create an environment hostile to microorganisms
around the site of injury or infection. If a drug targets and kills mast cells, then the
inflammation response would not occur, and the immune system would be less likely to
quickly fight the infection.
6. 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 produced after activation of B cells. Plasma cells
secrete huge number of antibodies. Memory cells linger after the infection subsides.
Upon subsequent contact with the same pathogen, memory cells rapidly transform into
plasma cells that produce antigen-specific antibodies.
7. Briefly explain the function of each innate and adaptive defense listed in figure 34.17.
Resident microorganisms prevent colonization by pathogens; white blood cells play many
roles in innate and adaptive defenses; macrophages engulf pathogens and present
antigens to helper T cells; skin blocks pathogens from entering the body; antimicrobial
proteins have many roles, such as directly killing pathogens; inflammation helps clear
debris and recruits immune cells; fever creates a hostile environment for pathogens;
helper T cells initiate and coordinate the adaptive immune response; plasma cells secrete
antibodies; antibodies inactivate pathogens and neutralize toxins; memory B cells initiate
the secondary immune response; cytotoxic T cells destroy infected or cancerous cells;
memory T cells help initiate the secondary immune response.
8. 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.
9. 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. Instead, a vaccine triggers the production of memory cells that can
quickly respond to a future infection.
10. Compare and contrast how a bacterial population becomes resistant to antibiotics and
how a person becomes immune to infections by a particular species of bacteria.
Bacterial resistance to antibiotics occurs by natural selection. Over many generations, the
bacteria that have genes that confer resistance to antibiotics become more common. In
contrast, a person can become immune to infections by a type of bacteria within his or
her lifetime. The first exposure to the bacteria triggers an immune response that produces
many memory cells. Upon second exposure to the bacteria, the memory cells trigger a
fast and strong immune response, so the bacteria may not become abundant enough to
produce a noticeable infection.
11. If worm infections suppress the immune system as suggested in section 34.6, 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 the worms would suppress.
12. 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.
13. 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.
14. Humans (and all other organisms) are in an evolutionary battle with a wide variety of
pathogens. How does natural selection favor (a) an immune system that adjusts to a
changing variety of pathogens and (b) pathogens that evade the immune system?
Natural selection favors a flexible immune system because a person who cannot defeat a
new type of pathogen may not survive long enough to reproduce. On the other hand,
natural selection favors pathogens that can evade the immune system because a pathogen
that cannot survive inside a host will not reproduce. The result is an evolutionary “arms
race” between pathogens and their hosts.
Pull it Together
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. Add memory B cells, plasma cells, memory cytotoxic T cells, primary immune
response, and secondary immune response to the concept map.
“B cells” leads with “differentiate into” to “plasma cells” and “memory B cells.”
“Cytotoxic T cells” leads with “differentiate into” to “memory cytotoxic T cells.” Both
types of memory cells lead with “initiate the” to “secondary immune response.”
“Macrophages” leads with “trigger the” to “primary immune response.” “Helper T cells,”
“cytotoxic T cells,” “B cells,” and “antibodies” lead with “participate in the” to “primary
immune response.”
3. Circle the immune system components that a vaccine activates.
A vaccine contains some or all of the same antigens as pathogens. Macrophages present
the antigens to helper T cells, which stimulates B cells that produce antibodies specific to
the antigen.
4. How do lymph and lymph nodes fit into this concept map?
After a macrophage consumes a pathogen, it travels in lymph to a lymph node, where
activated helper T cells trigger the adaptive immune response.
5. Where else might macrophages fit in this concept map?
Macrophages could be listed along with the other innate defenses.