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Immunity and
Health
PATHOGENS
Viruses
Bacteria
Fungi
Parasitic protists
Parasitic worms
The immune system protects us from a diverse
group of pathogens—disease-causing viruses and
microorganisms, often referred to as “germs.”
FIGURE 26-3 Examples of the foreign, “non-self” microbes and viruses that can harm the body
© 2012 W. H. Freeman and Company
1
WHAT IS LIFE? A GUIDE TO BIOLOGY, SECOND EDITION
DIVISIONS OF THE IMMUNE SYSTEM
Skin wound
PHYSICAL BARRIERS
• Form a nearly impenetrable wall, keeping
pathogens from entering
body tissues
• Consist of skin, mucous
membranes, and their
associated anti-pathogen
secretions
NON-SPECIFIC IMMUNITY
• Recognizes and destroys
pathogens that breach
external barriers
• Responds to all pathogens in the same way
• Responds to infection
within minutes
SPECIFIC IMMUNITY
• Destroys pathogens that
are not killed by nonspecific defenses
• Recognizes specific
pathogens and forms a
memory of each
• Responds to infection in
hours to days
Pathogens
Skin
Pathogens
Non-specific
immune system
cells
Pathogens
Specific immune
system cells
After the specific immune system forms a “memory” of a pathogen,
it fights off the same pathogen more quickly in the future, often
resulting in lifelong protection from that pathogen.
FIGURE 26-4 Layered defenses of the immune system block entry and fight invaders
THE INTEGUMENTARY SYSTEM
Hair
Nails
Skin surface
Hair
Oil glands
Sweat glands
Fat cells
Nerve fiber
Blood vessel
The integumentary system protects you from
pathogens and external stresses such as
friction, pressure, sunlight, and dehydration.
FIGURE 26-5 Providing a nearly impenetrable barrier: the integumentary system
2
© 2012 W. H. Freeman and Company
CHAPTER 26 • Immunity and Health
DEFENSE MECHANISMS OF THE
INTEGUMENTARY SYSTEM
SKIN
This forms a nearly impenetrable
barrier that keeps pathogens from
entering the body.
LYSOZYME AND OTHER ENZYMES
Lysozyme in saliva and tears,
and digestive enzymes in the
small intestine, kill many bacteria.
ACIDIC SECRETIONS
Stomach acids, acidic vaginal
secretions, and acidic urine, all protect
the digestive, reproductive, and urinary
tracts from bacterial pathogens.
CILIA
Hair-like extensions on the surface of the
respiratory tract move mucus-entrapped
pathogens up and out of the lungs.
TEARS
Fluid containing antiviral and
antibacterial chemicals washes away
microorganisms from around the eyes.
EAR WAX
This sticky substance can trap
microorganisms in the ear canal.
FIGURE 26-7 Security barrier
RESPONDING TO PATHOGENS IN THE BODY
Pathogens
Pathogens
Immune system cells
Immune
system cells
Cytokines
Pathogens
1
Immune system cells
IDENTIFY THE INTRUDER
Non-specific immune system
cells recognize molecules found
only on the surface of pathogens and bind to them, marking
the pathogens as invaders.
2
CALL FOR BACKUP
Immune system cells secrete
signaling proteins called
cytokines that recruit more
immune cells to the site of the
infection or warn other cells to
protect themselves.
3
ATTACK AND REMOVE
Specialized immune system
cells destroy, break down, and
ingest both the pathogens and
any cells they’ve infected.
FIGURE 26-8 Sometimes the security measures fail
© 2012 W. H. Freeman and Company
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WHAT IS LIFE? A GUIDE TO BIOLOGY, SECOND EDITION
THE WHITE BLOOD CELLS OF NON-SPECIFIC IMMUNITY
The non-specific immune system consists of several types of
white blood cells that are made in the bone marrow and
released into the bloodstream.
NEUTROPHILS
• Phagocytic cells that ingest small
organisms, primarily bacteria
• Destroy both the pathogen and
themselves in the process
• Nucleus is multi-lobed
Pathogens
MACROPHAGES
• Phagocytic cells that ingest whole
pathogens as well as large debris
such as dead cells
• Present pieces of pathogens on
their surface, advertising the
infection to cells of the specific
immune system
Presented pathogen fragments
DENDRITIC CELLS
• Phagocytic cells (named for their
tentacle-like arms, called
dendrites) that present ingested
pathogens to cells of the specific
immune system
Infected cell
NATURAL KILLER (NK) CELLS
• Kill body cells infected by
pathogens by poking holes in the
cell membranes
• Also play a role in recognizing
and killing cancer cells
FIGURE 26-9 Hardworking cells
COMPLEMENT PROTEINS
A collective group of circulating proteins, called
complement proteins, recognize pathogens and help fight
them in various ways, including by sticking to them and
increasing the ability of phagocytes to consume them,
and by directly destroying them (shown here).
Pathogen
Complement proteins
Water
Cell
membrane
Some complement proteins blast holes in the
membranes of pathogens, allowing water to rush
inward and causing the pathogen to burst.
FIGURE 26-11 Defensive proteins “complement” the other cells of the non-specific system to combat pathogens
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© 2012 W. H. Freeman and Company
CHAPTER 26 • Immunity and Health
Histamine-containing
granules
Cell membrane
Mast cell nucleus
FIGURE 26-13 A mast cell
THE INFLAMMATORY RESPONSE
Skin surface
Interstitial fluid
Pathogens
Macrophages
1
1
After you cut your finger with a knife,
invading pathogens from the knife and
the skin’s surface enter your body.
2
Macrophages residing in tissues
surrounding the cut begin engulfing
the pathogens and releasing cytokines,
recruiting more phagocytes and other
white blood cells to the area.
3
2
Blood vessel
Basophils, which circulate in the blood,
and mast cells, found in the tissues,
trigger the inflammatory reaction by
releasing histamine.
4
Histamine causes nearby non-injured
blood vessels to dilate, increasing the
flow of blood and allowing for a greater
supply of defensive molecules and cells
that can fight infection.
5
Histamine also causes the blood vessels to
become more leaky, allowing neutrophils
to more easily exit the blood and enter
the site of infection.
6
Cytokines
The inflammatory response continues
until the pathogens have been eliminated
and the skin grows back, forming an
impenetrable barrier once again.
Basophils
Mast cells
3
4
5
Neutrophils
Histamine
6
Inflammatory response causes redness,
heat, swelling, and pain, but ultimately
leads to tissue healing.
FIGURE 26-14 Reacting to injury
© 2012 W. H. Freeman and Company
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WHAT IS LIFE? A GUIDE TO BIOLOGY, SECOND EDITION
EXPOSURE LEADS TO IMMUNITY
GETTING SICK
Exposure to a pathogen causes the body to form a
memory of the pathogen, producing the cells necessary
to rapidly respond to the pathogen if encountered
again. Shown here: a toddler with chicken pox.
GETTING A VACCINATION
Exposure to a weakened or harmless form of a pathogen
in a vaccine allows the body to form a memory of the
pathogen without the risk of symptoms. The body then
produces the cells necessary to rapidly respond to the
pathogen if encountered again.
FIGURE 26-16 Two paths to immunity: contracting and fighting an illness, or receiving a vaccine
ANTIGENS AND ANTIBODIES
ANTIGENS
Foreign substances that
induce a specific immune
response
A single pathogen
often contains many
different antigens.
ANTIBODIES
Proteins that recognize certain antigens, enhancing
the non-specific system’s ability to recognize and
destroy those antigens
FIGURE 26-17 Antibodies are proteins that recognize foreign molecules called antigens
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© 2012 W. H. Freeman and Company
CHAPTER 26 • Immunity and Health
THE INFLUENZA VIRUS IS CONSTANTLY CHANGING
Inluenza virus
Antigens
Time
Strains of the influenza virus are constantly changing, so
the body encounters a slightly different form with each
new flu season, and therefore a different set of antigens.
FIGURE 26-18 Because the influenza virus changes so rapidly, flu vaccinations need to be given annually
ANTIBODY STRUCTURE
Antigen
Variable region
(antigen-binding
site)
Constant region
Antibodies—always with the same “Y”
shape—can be released by B cells as freefloating sentinels within the body fluids or
embedded within the surface membrane
of a B cell.
Antibodies all have the same general
Y-shaped structure, but variability in one
part of it makes each antibody unique
and able to recognize a specific antigen.
FIGURE 26-20 Variations on a molecular theme
© 2012 W. H. Freeman and Company
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WHAT IS LIFE? A GUIDE TO BIOLOGY, SECOND EDITION
ANTIBODY FUNCTIONS
Antibodies function in several ways to help destroy
pathogens and free-floating antigens.
PHAGOCYTE SIGNALING
Antibodies bind to antigens on the
surface of pathogens, making it
easier for phagocytes to find the
pathogens and destroy them.
ANTIGEN CLUMPING
Antibodies make pathogens and
free-floating antigens clump
together, making it easier for
phagocytes to find them and
destroy them.
PREVENTION OF CELL ENTRY
Antibodies coating the surface of
pathogens prevent the pathogens
from entering body cells.
COMPLEMENT PROTEIN SIGNALING
Antibodies recruit complement
proteins, which poke holes in
pathogen membranes, causing the
pathogen cells to burst.
FIGURE 26-21 Antibodies fight pathogens in several ways
THE WHITE BLOOD CELLS OF SPECIFIC IMMUNITY
B cells and T cells are responsible for the specific immunity
response. They are named for the location in the body where they
mature (the bone marrow and thymus).
B CELLS
• Develop and mature in bone marrow
• Lymphocytes that combat pathogens
by releasing antibodies into body
fluids when antigens are detected
Bone marrow
Antigen receptors
T CELLS
• Develop in bone marrow and mature
in the thymus
• Lymphocytes that combat pathogens
by directly destroying the infected
cells
Bone marrow
Thymus
FIGURE 26-22 B cells and T cells
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© 2012 W. H. Freeman and Company
CHAPTER 26 • Immunity and Health
SPECIFIC IMMUNE SYSTEM RESPONSES
Pathogens
Antigens
Pathogens
Antibodies
Antigens
B cells
T cells
Antigen
receptors
Antigen
receptors
HUMORAL IMMUNITY
• Protection against pathogens and toxins found in
body fluids, such as blood and lymph
• Carried out by B cells that secrete antibodies into
body fluid, making it easier for phagocytes to engulf
and destroy invading pathogens
Infected cell
CELL-MEDIATED IMMUNITY
• Protection against pathogens and toxins found within
body cells
• Carried out by T cells that directly destroy invading
pathogens as well as the infected cells
FIGURE 26-23 Fighting invaders in body fluids and within cells
THE PRIMARY RESPONSE TO INFECTION
Antigens
1 RECOGNITION
When a lymphocyte comes into
contact with the antigen specific
to its receptor, the cell initiates a
response that leads to the
destruction of the antigen.
Antigen
receptor
Lymphocyte
2 CLONAL SELECTION
The lymphocyte divides numerous
times, creating two populations of
cells with the same antigen
specificity.
3 EFFECTOR CELLS ATTACK
Effector cells immediately
take action, leading to the
destruction of the antigen.
Effector cells
4 MEMORY CELLS REMEMBER
Memory cells remember the antigen
so that if the body is infected with
the same antigen in the future, they
will be ready to respond.
Memory cells
The primary response leads to destruction of an
antigen and generation of memory cells to fight
the antigen should it ever be encountered again.
FIGURE 26-24 Fighting now and later
© 2012 W. H. Freeman and Company
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WHAT IS LIFE? A GUIDE TO BIOLOGY, SECOND EDITION
THE SECONDARY RESPONSE TO INFECTION
Memory cells produced during a primary immune response (such as to
chicken pox) enable you to mount a faster, stronger secondary response
following exposure to the virus later in life, preventing illness.
Antibody levels
Secondary response
Primary response
Exposure to chicken pox
Exposure to chicken pox
Time
FIGURE 26-25 Another encounter with the same antigen
T CELL-MEDIATED RESPONSE
Antigenpresenting cell
(dendritic cell)
1 PRESENTATION AND RECOGNITION
An antigen-presenting cell displays
digested particles of a virus to a
helper T cell that recognizes the viral
antigen being presented.
Viral antigen
Helper
T cell
Cytokines
2 ACTIVATION
Binding to the antigen-presenting
cell causes the helper T cell to
produce cytokines, activating
cytotoxic T cells (as well as the B
cells of the humoral response).
Activated cytotoxic T cell
3 CLONAL EXPANSION
Both helper T cells and cytotoxic
T cells undergo clonal expansion,
producing vast amounts of
memory and effector cells with
specificity for the viral antigen.
Cytotoxic T cells
(memory and
effector cells)
Helper T cells
(memory and
effector cells)
Cytokines
4 MATURATION
Other cytokines produced by the
helper T cells make the cytotoxic
T cells mature and ready to fight
the pathogen.
Mature cytotoxic
T cells
Infected cell
5 DESTRUCTION
Mature cytotoxic T cells circulate
throughout the body, destroying
cells infected with the specific
viral antigen.
FIGURE 26-26 Helper T cells and cytotoxic T cells work together to recognize and respond to pathogens
10
© 2012 W. H. Freeman and Company
CHAPTER 26 • Immunity and Health
AUTOIMMUNITY: TYPE 1 DIABETES
Type 1 diabetes is an autoimmune disorder in which
cytotoxic T cells destroy one’s own pancreatic cells.
Healthy pancreatic cells
Cytotoxic T cells
T cell receptors incorrectly recognize healthy
pancreatic cells as antigens, initiating a cellmediated immune response against them.
FIGURE 26-28 Type 1 diabetes is an all-too-common autoimmune disorder
IMMUNE SYSTEM DEFICIENCY: HIV
HIV virus
ACUTE PHASE
1
2
HIV infects helper T cells,
macrophages, and
dendritic cells.
Helper Macrophage Dendritic
T cell
cell
Humoral and cell-mediated
immune responses
eliminate over 99% of the
virus.
Antibodies against HIV, made in the humoral
response, can be detected in blood tests
within months of the initial infection.
CHRONIC PHASE
3
Viruses not destroyed by the
primary response can lie
dormant inside infected helper T
cells or replicate and infect new
helper T cells.
4
Infected helper T cells are
destroyed by the viral infection
or by the immune system’s
normal responses.
5
New helper T cells arise, yet
new viruses infect them.
Eventually, the number of
viruses climbs and the
number of helper T cells
decreases.
A severe shortage of helper T cells marks the
progression from HIV infection to AIDS.
AIDS
6
Without helper T cells, B
cells and cytotoxic T cells
cannot be activated.
B cell
Helper T cell
7
Common pathogens that
are normally kept at bay in
a healthy individual lead to
serious illness and,
ultimately, death.
Cytotoxic T cell
Common pathogens
FIGURE 26-30 The progression from HIV infection to AIDS
© 2012 W. H. Freeman and Company
11
WHAT IS LIFE? A GUIDE TO BIOLOGY, SECOND EDITION
HIV/AIDS CASES AROUND THE WORLD
ADULTS INFECTED WITH HIV (%)
<0.1
0.1–0.49
0.5–0.9
1.0–4.9
5.0–14.9 15.0–28.0 No data
FIGURE 26-32 HIV/AIDS is widespread in sub-Saharan Africa
ALLERGIC REACTION
FIRST RESPONSE
1
2
4
Plasma cell
Antibodies
Mast cells become
“sensitized” to the allergen
after the antibodies bind to
their surface.
SECOND RESPONSE
3
Allergen
When the body first
encounters an allergen,
although the material (such
as peanut proteins) is harmless, a humoral response
occurs: memory cells form
and plasma cells secrete
antibodies specific to the
allergen.
Mast cell
Allergen
Allergens bind to the
antibodies that are still
attached to the mast cells,
causing the mast cells to
release histamine.
Mast cell
Histamine
Histamine causes the
blood vessels to dilate
and become leaky, and
inflammation ensues.
Allergies result from overly sensitized
mast cells releasing histamine and
cytokines in response to allergens.
FIGURE 26-33 Allergens induce a humoral response in some individuals
12
© 2012 W. H. Freeman and Company