Immune System
A. Basic Function
1. Agents of Infectious Diseases (virus, bacteria, fungi, parasites)
2. Spread of Diseases
Direct contact
A. Person to person:
Bacteria, viruses or other germs can pass from one person to another through physical
contact, coughing or kissing. These germs can also spread through the exchange of body fluids
from sexual contact or through a blood transfusion.
B. Animal to person:
A household pet might seem harmless, but pets can carry many germs. Being bitten or
scratched by an infected animal can make you sick and, in extreme circumstances, could even
cause death. Handling animal waste can be hazardous, too. For example, you can acquire a
toxoplasmosis infection by scooping your cat's litter box, particularly if you're pregnant.
C. Mother to unborn child:
A pregnant woman may pass germs that cause infectious diseases to her unborn baby.
Germscan pass through the placenta, as is the case of the AIDS virus and the toxoplasmosis
parasite.Or germs can spread during labor and delivery, as is the case for a mother infected with
group B streptococcus.
Indirect contact
Disease‐causing organisms are found on inanimate objects, such as tabletops, doorknobs or
faucet handles. When you touch a doorknob after someone with the flu or a cold has
touched it, you can pick up the germs he or she left behind. If you then touch your eyes,
mouth or nose before washing your hands, you may become infected. There are some
organisms that naturally live in the environment that carry infectious disease; these include
fungal infections like histoplasmosis or blastomycosis, as well as bacterial infections such as
anthrax.
Airborne dispersal
A. Droplet transmission:
When you cough or sneeze, you expel droplets into the air around you. If you're sick with the flu
or other illness the droplets you expel contain the germ that caused your illness. Droplets travel
only about three feet because they're usually too large to stay suspended in the air for a long
time. However, if a droplet from an infected person comes in contact with your eyes,
nose or mouth, you may soon experience symptoms of the illness. Crowded, indoor
environments may promote the chances of droplet transmission ‐‐which explains the increase in
respiratory infections during the winter months.
B. Particle transmission:
Some disease‐causing germs travel through the air in particles considerably smaller than
droplets. These tiny particles remain suspended in the air for extended periods of time and
can travel in air currents. If you breathe in an airborne virus, bacterium or other germ, you
may become infected and show signs and symptoms of the disease. Colds caused by viruses,
influenza and tuberculosis are examples of infectious diseases that spread through the air in
both particle and droplet form.
Infectious diseases spread through insect bites and vehicles
A. Insect bites and stings:
Some germs use insect carriers — such as mosquitoes, fleas, lice or ticks ‐‐ to move
from host to host. Mosquitoes can carry the malaria parasite or West Nile virus, and deer ticks
may carry the bacterium that causes Lyme disease. The insect carries the germ on its body
or in its intestinal tract. After the insect lands on you or bites you, the germs move into your
body and can make you sick. Sometimes the germs that cause infectious disease need the
insect for specific biological reasons. They use the insect's body to multiply, which is
necessary before the germs can infect a new host.
B. Food contamination:
Another way disease‐causing germs can infect you is through contaminated food and water.
Contamination with Escherichia coli (E. coli) is common. E. coli is a bacterium present in or
on certain foods, such as undercooked hamburger or unwashed fruits or vegetables.
When you eat foods contaminated with
B. Types of Responses
1. Nonspecific Immune Response
a. Skin
b. Secretions (e.g. tears)
c. Inflammatory Response
d. Fever
The body possesses many mechanisms that impart nonspecific defense. The objectives
of these mechanisms are to prevent microorganisms from gaining a foothold in the
body and to destroy them if they penetrate to the deeper tissues.
1.Mechanical barriers. Mechanical barriers at the portal of entry represent the first
line of defense for the body. These defenses are normally part of the body's anatomy
and physiology. The skin is a representative example. The outermost layers of skin
consist of compacted, cemented cells impregnated with the insoluble protein keratin.
The thick top layer is impervious to infection and water. In the unbroken state, it
usually is not penetrated by pathogens.
2.The mucous membranes of the urinary, respiratory, and digestive tracts are
another example. They are moist and permeable, but their fluids, such as tears,
mucus, and saliva, rid the membrane of irritants. Nasal hairs trap particles in the
respiratory tract, and the fluids exert a flushing action. Cilia on the cells sweep and
trap particles in the respiratory tract, and coughing ejects irritants.
3.Chemical defenses. Among the nonspecific chemical defenses of the body are the
secretions of lubricating glands. The tears and saliva contain the enzyme lysozyme,
which breaks down the peptidoglycan of the cell wall of Gram-positive bacteria. The
lactic acid of the vagina imparts defense, and the extremely caustic hydrochloric
acid of the stomach is a barrier to the intestine. Semen contains the antimicrobial
substance spermine that inhibits bacteria in the male urogenital tract.
4.Genetic barriers. The hereditary characteristics of an individual are a deterrent to
disease as well. For example, humans suffer HIV infection because their Tlymphocytes have the receptor sites for the human immunodeficiency virus. Dogs,
cats, and other animals are immune to this disease because they do not possess the
genes for producing the receptor sites. Conversely, humans do not suffer canine
distemper because humans lack the appropriate receptor sites for the virus that
causes the disease.
5.Inflammation. Inflammation is a nonspecific response to any trauma occurring to
tissues. It is accompanied by signs and symptoms that include heat, swelling,
redness, and pain. Inflammation mobilizes components of the immune system, sets
into motion repair mechanisms, and encourages phagocytes to come to the area and
destroy any microorganisms present.
6.Inflammation can be controlled by nervous stimulation and chemical substances
called cytokines. These chemical products of tissue cells and blood cells are
responsible for many of the actions of inflammation. The loss of fluid leads to a local
swelling called edema. In some types of inflammation, phagocytes accumulate in the
whitish mass of cells, bacteria, and debris called pus.
Summary of Non-Specific
There are physical, chemical, and cellular defenses against invasion by viruses, bacteria, and
other agents of disease.
During the early stages of an infection, there is an inflammatory response
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Non-specific attack
Phagocytes active ("eat" pathogen)
During later stages, leucocytes produce immune responses
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Antigen - a foreign substance which triggers an immune response
Some WBC's produce antibodies in huge amounts
o Antibodies - substances which bind to specific antigens and tag them for
destruction
o Other WBC's (executioner cells) directly destroy body cells
Surface coverage - the first line of defense
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The body is protected from pathogens by the skin and mucous membranes
o Skin - dead cellular layer - dry, low pH
o Mucous membranes contain lysozymes (enzymes which break down bacteria)
o Other cells contain cilia which filter pathogens and particulates
Breaks in the protective barrier
o Digestive openings
o Reproductive openings
o Respiratory openings
o Sensory Organs
2. Specific Immune Response
a. Humoral Immunity
b. Cell-Mediated Immunity
Humoral and Cell-Mediated Immune
Responses
The immune system distinguishes two
groups of foreign substances. One group
consists of antigens that are freely
circulating in the body. These include
molecules, viruses, and foreign cells. A
second group consists of self cells that
display aberrant MHC proteins. Aberrant
MHC proteins can originate from antigens
that have been engulfed and broken
down (exogenous antigens) or from
virus-infected and tumor cells that are
actively synthesizing foreign proteins
(endogenous antigens). Depending on the
kind of foreign invasion, two different
immune responses occur:
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The humoral response (or antibodymediated response) involves B cells
that recognize antigens or
pathogens that are circulating in
the lymph or blood (“humor” is a
medieval term for body fluid). The
response follows this chain of
events:
1. Antigens bind to B cells.
2. Interleukins or helper T cells
costimulate B cells. In most
cases, both an antigen and a
costimulator are required to
activate a B cell and initiate B
cell proliferation.
3. B cells proliferate and produce
plasma cells. The plasma cells
bear antibodies with the identical
antigen specificity as the antigen
receptors of the activated B cells.
The antibodies are released and
circulate through the body,
binding to antigens.
4. B cells produce memory cells.
Memory cells provide future
immunity.
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The cell-mediated response involves mostly T cells and responds to any cell that
displays aberrant MHC markers, including cells invaded by pathogens, tumor cells,
or transplanted cells. The following chain of events describes this immune
response:
1. Self cells or APCs displaying foreign antigens bind to T cells.
2. Interleukins (secreted by APCs or helper T cells) costimulate activation of T
cells.
3. If MHC-I and endogenous antigens are displayed on the plasma membrane,
T cells proliferate, producing cytotoxic T cells. Cytotoxic T cells destroy cells
displaying the antigens.
4. If MHC-II and exogenous antigens are displayed on the plasma membrane,
T cells proliferate, producing helper T cells. Helper T cells release
interleukins (and other cytokines), which stimulate B cells to produce
antibodies that bind to the antigens and stimulate nonspecific agents (NK
and macrophages) to destroy the antigens.
Non-specific responses - the second line of defense
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Non-specific responses are generalized responses to pathogen infection - they do not
target a specific cell type
The non-specific response consist of some WBC's and plasma proteins
Phagocytes - cells which "eat" foreign material to destroy them
o Phagocytes are formed from stem cells in bone marrow (stem cells are
undifferentiated WBC's)
 Neutrophil - phagocytize bacteria
 Eosinophils - secrete enzymes to kill parasitic worms among other
pathogins
Macrophage - "big eaters" phagocytize just about anything
Macrophage destroying bacterial cells
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Non-phagocytic leucocytes o Basophil - contain granules of toxic chemicals that can digest foreign
microorganisms. These are cells involved in an allergic response
o
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Mast Cells - similar to basophils, mast cells contain a variety of inflammatory
chemicals including histamine and seratonin. Cause blood vessels near wound to
constrict.
Complement proteins - plasma proteins which have a role in nonspecific and specific
defenses
o
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Form a cascade effect - if only a few are activated, they will trigger others to
become active in great numbers
 Some punch holes in bacterial walls (forms holes where cellular
components leak out)
 Some promote inflammation
 Concentration gradients attract phagocytes to irritated or damaged
tissue
 Encourage phagocytosis in phagocytes (promotes "eating")
 Some bind to the surface of invading organisms
Chemokines - create a chemical gradient to attract neutrophils and other leucocytes to
the wound site
Inflammation
o
o
o
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Causes localized redness, swelling, heat, and pain
Changes in capillary wall structure allow interstitial fluid and WBC's to leak out
in tissue
Promotes macrophage (phagocytic WBC's) activity
Macrophages secrete Interleukins (communication proteins among WBC's)
 Interleukin-1: increases body temperature (i.e. causes a fever)
 This enhances the WBC's ability to protect the body
 Causes drowsiness - reduces the body's energy usage and stress
The Immune System (Specific Responses) - the third line of defense
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Called into action when nonspecific methods are not enough and infection becomes
widespread
Types of cells involved in the immune system:
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Macrophages - engulf foreign objects
o Inform T lymphocytes at a specific antigen is present
Helper T cells - produce and secrete chemicals which promote large numbers of effector
and memory cells
Cytotoxic T cells - T lymphocytes that eliminate infected body cells and tumor cells
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B cells - produce antibodies (secrete them in the blood or position them on their cell
surfaces)
Each type of virus, bacteria, or other foreign body has molecular markers which make it unique
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Host lymphocytes (i.e. those in your body) can recognize self proteins (i.e. those which
are not foreign)
When a nonself (foreign) body is detected, mitotic activity in B and T lymphocytes is
stimulated
o While mitosis is occurring, the daughter populations become subdivided
 Effector cells - when fully differentiated, they will seek and destroy
foreign
 Memory cells - become dormant, but can be triggered to rapid mitosis if
pathogen encountered again
Thus, immunological specificity and memory involve three events:
(1) Recognition of a specific invader
(2) Repeated cell divisions that form huge lymphocyte populations
(3) Differentiation into subpopulations of effector and memory cells
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Antigen - a nonself marker that triggers the formation of lymphocyte armies
Antibodies - molecules which bind to antigens and are recognized by lymphocytes
Antigen-presenting cell - a macrophage which digests a foreign cell, but leaves the antigens
intact. It then binds these antigens to MHC molecules on its cell membrane. The antigen-MHC
complexes are noticed by certain lymphocytes (recognition) which promotes cell division
(repeated cell divisions)
Molecular cues that stimulate lypmphocytes to create an immune response
T cells (Helper T cells and Cytotoxic T cells)
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T cells arise from stem cells in the bone marrow - they then travel to the thymus where
the differentiate and mature. At maturity, they acquire receptors for self markers (MHC
molecules) and for antigen-specific receptors. They are then released into the blood as
"virgin" T cells.
T cells ignore other cells with MHC molecules and they ignore free-floating antigens.
However, they will bind with a antigen-presenting macrophage (a macrophage possessing
a MHC-antigen complex). This binding promotes rapid cell division and differentiation
into effector and memory cells (all with receptors for the antigen)
Effector helper T cells secrete interlukins (stimulate both T and B cells to divide and
differentiate)
Effector cytotoxic T cells recognize infected cells with the MHC-antigen complex. They
then destroy the cell with perforans (enzymes which perforate the cell membrane,
allowing cytoplasm to leak out) and other toxins which attack organelles and DNA
Cell-mediated immune response
B cells and Antibodies
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B cells also arise from stem cells in the bone marrow. As they develop and mature, they
start synthesizing a single type of antibody
Antibodies are proteins which recognize antigens
The virgin B cell produces antibodies which move to the cell surface and stick out
The B cell floats in the blood - when it encounters the specific antigen it becomes primed
for replication
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The B cell must receive an interleukin signal from a helper T cell which has already
become activated by a macrophage with a MHC-antigen complex. This promotes rapid
cell division.
The B cell population then differentiates into effector and memory B cells
The effector B cells then produce a staggering amount of free-floating antibodies
o When these free-floating antibodies encounter an antigen, they tag it for
destruction by phagocytes and complementary proteins
o These types of responses are only good for extracellular toxins and pathogens they cannot detect pathogens or toxins located inside of a cell
Antibody-mediated immune response
Where do all of these interactions take place? - in the lymph nodes.
3.
C. Human Health and Disease Transmission
1. Genetic Risks
2. Environmental Factors
3. Pathogens
4. Immune System Disorders
D. Fighting Infectious Diseases
1. Antibiotics
2. Vaccines (How Vaccines Work )
3. Genetic Engineering
The Diseases Vaccines Prevent and How
Acquired Immunity: immunity to a particular disease that is not innate but has been acquired during life;
immunity can be acquired by the development of antibodies after an attack of an infectious disease or by a
pregnant mother passing antibodies through the placenta to a fetus or by vaccination
Artificial immunity is classified as active and comes in the form of vaccinations, typically give to children
and young adults. The passive form of artificial immunity involves introducing an antibody into the system
once a person has already been infected with a disease, ultimately relieving the present symptoms of the
sickness and preventing re-occurrence.
Passive Immunity: an impermanent form of acquired immunity in which antibodies against a disease are
acquired naturally (as through the placenta to an unborn child) or artificially (as by injection of antiserum)
What Diseases Do Vaccines Prevent?
Vaccines are the safest way to protect you, your children and your community from a long list of
serious and potentially life-threatening illnesses. You can read about many of the diseases
vaccines are used to prevent below:
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Chickenpox
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Diphtheria
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Hepatitis A
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Human Papillomavirus (HPV, genital or venereal warts)
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Measles
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Mumps
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Pneumococcal Disease
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Rubella
Haemophilus Influenza Type B
Hepatitis B
Influenza
Meningococcal Disease
Pertussis (Whooping cough)
Poliomyelitis
Shingles
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Tetanus
For a full list of all the diseases vaccines prevent, visit the CDC website.
How Do Vaccines Prevent Disease?
Vaccines protect you by preparing your immune system to recognize and fight serious, and
sometimes deadly, diseases.
A vaccine contains a specific part of a germ (bacteria or virus), called an antigen. The antigen is
killed or disabled before it's used to make the vaccine, so it can't make you sick. Vaccines, and
the antigens they contain, stimulate your immune system's B cells to develop protective
substances called antibodies. These antibodies are responsible for killing germs that enter your
body. Once activated, B cells can stay in your body for a lifetime and allow your body to
remember the germ that stimulated their creation. Throughout your life, these cells will
recognize and fight the actual disease caused by the germ when and if you come into contact
with it.
If you're interested in reading about this topic further, visit the CDC website.
Vaccines help prevent infectious diseases and save many lives. Because of vaccines, diseases
including polio and measles, that once took devastating tolls on families and whole communities,
are now almost extinct.
 Compare and contrast types of infectious agents that may infect the human body, including viruses, bacteria, fungi
and parasites.
 Identify and explain the basic functions of the human immune system, including specific and nonspecific
immune responses.
 Describe how the human immune system responds to vaccines and/or antibiotics.
 Explain the significance of genetic factors, environmental factors and pathogenic agents to health from the
perspective of both individual and public health.
 Describe ways human beings protect themselves from hazardous weather and sun exposure.
 Analyze how heredity and family history can impact personal health.
 Analyze strategies for prevention, detection, and treatment of communicable and chronic diseases.