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The Immune System
In this lesson, you will Learn About…
 The function of the immune system.
 How antibodies protect the body from
pathogens.
 How you can develop immunity to a disease.
The Immune System
The Vocabulary terms in this lesson are:
 Immune system.
 Inflammation.
 Lymphatic system.
 Lymphocytes.
 Antigens.
 Antibodies.
 Immunity.
 Vaccine.
Keeping Pathogens Out
Each day you are exposed to countless
pathogens.
Your body can repel, trap, or destroy most
of these pathogens before they can do any
damage.
The five major barriers against such
invaders help keep pathogens out.
The Five Major Barriers
Tears
Tears wash away
pathogens. They
also contain
chemicals that kill
some harmful
organisms.
Mucous Membranes
Saliva in your
mouth destroys
many harmful
organisms.
These tissues line
your mouth, nose,
throat, eyes, and
other body openings.
They trap pathogens.
When you cough or
sneeze, you expel the
pathogens trapped by
the mucous
membranes in your
nose and throat.
Skin
Stomach Acid
Saliva
Your skin acts
as a protective
barrier. Pathogens
may get through
this barrier when
you have a cut,
burn, or scrape.
The acid in your
stomach
destroys many
pathogens.
The Immune System’s Nonspecific
Response
If pathogens enter the body, the immune
system launches an attack.
The immune system’s nonspecific response
is always the same, regardless of the type
of foreign substance that invades the body.
The inflammatory response is a nonspecific
response that occurs if a foreign invader
gets past the five major barriers.
The Immune System’s Nonspecific
Response (cont’d.)
The steps of the inflammatory response:
1. The blood supply to the affected area
increases, and circulation in that area slows
down.
2. This raises the blood pressure in the area,
causing fluid to leak from the blood vessels.
3. As a result, surrounding tissues swell.
4. Special white blood cells called phagocytes
attack the invading pathogens.
5. The phagocytes surround the invaders, take
them apart, and “eat” them.
The Immune System’s Nonspecific
Response (cont’d.)
Other components of the inflammatory
response:
 The phagocytes release special proteins to
help defend the body. One of these proteins,
interferon, stops viruses from reproducing
and helps the cells that fight infection.
 If the infection has spread throughout the
body, a fever becomes part of the
inflammatory response.
Inflammation
During the inflammatory response, more
blood flows to the infected area, and
phagocytes rush in to destroy the invading
pathogens.
Phagocyte
Pathogen
Splinter
Pathogen
Phagocyte
Blood vessel
The Immune System’s Specific
Responses
When pathogens survive the inflammatory
response, the body counters with a
specific response. This response:
 Is tailored to a particular pathogen and the
poisons that it produces.
 Not only defends the body against an invading
pathogen, but also allows the immune system
to “remember” that particular type of
pathogen.
The Lymphatic System
The lymphatic system is a secondary
circulatory system that helps your body
fight pathogens and maintain its fluid
balance.
Your body calls upon the lymphatic system
to fight against specific pathogens.
The Lymphatic System (cont’d.)
The lymphatic system circulates a watery
fluid known as lymph.
Special white blood cells in the lymph are
called lymphocytes. There are three types:
 B cells
 T cells
 NK cells
The Lymphatic System (cont’d.)
The lymph also contains phagocytes called
macrophages that digest and process the
invading pathogens and then help the
lymphocytes identify them.
The three types of lymphocytes are
activated when the body recognizes a part
of a pathogen known as an antigen.
Antibodies
Your body reacts to antigens by producing
antibodies. Antibodies are proteins that
attach to antigens, keeping them from
harming the body.
B cells produce specific antibodies to fight
a particular type of antigen.
The Immune System’s Response to
Infection
2.A macrophage
engulfs one of the
pathogens and
presents its
antigens to the T
cells, which
identify the
pathogen as an
invader.
1. A pathogen invades the
body. The antigens in the
pathogen activate the
immune system’s response.
3. T cells multiply
at the site of the
infection. The T
cells activate the
B cells.
The Immune System’s Response to
Infection (cont’d.)
7.Memory T cells and B cells remain
in the system. The next time this
type of pathogen enters the
system, these memory cells attack
it swiftly.
4.B cells multiply.
6.The B cells
release
antibodies,
which attach
themselves to
the antigens.
Special T
cells attack
and destroy
the invaders.
5.Some B
cells and T
cells
become
memory
cells.
Immunity
Your body’s ability to resist the pathogens
that cause a particular disease is called
immunity.
You were born with some immunities that
were passed on to you from your mother.
These immunities lasted for a few months,
after which you began to produce
antibodies on your own.
Immunity (cont’d.)
Another way your body builds immunity is
by being exposed to pathogens and by
having certain diseases.
When your body encounters an antigen, it
produces memory B cells and T cells.
Often these memory cells stay in the blood.
If the same type of pathogen returns, the
immune system fights it so that you do not
become ill a second time.
Immunity (cont’d.)
You don’t have to get a disease to acquire
immunity to it. You can receive an
immunization, or vaccine.
This process is called immunization
because the antibodies your body produces
in response to the vaccine will build
immunity.
Reviewing Terms and Facts
1. Define the term immune system.
The immune system is a combination of body
defenses made up of cells, tissues, and organs
that fight off pathogens.
Reviewing Terms and Facts
2. What are the body’s five major barriers
against pathogens?





Tears
Saliva
Skin
Mucous membranes
Stomach acid
Thinking Critically
3. Why do you think diseases that damage
the immune system are so dangerous to
the body?
Vocabulary Review
The immune system is a combination of
body defenses made up of cells, tissues,
and organs that fight off pathogens.
Vocabulary Review
Inflammation is the body’s response to
injury or disease, resulting in a condition of
swelling, pain, heat, and redness.
Vocabulary Review
The lymphatic system is a secondary
circulatory system that helps the body fight
pathogens and maintain its fluid balance.
Vocabulary Review
Lymphocytes are special white blood cells
in the lymph.
Vocabulary Review
Antigens are substances that send the
immune system into action.
Vocabulary Review
Antibodies are proteins that attach to
antigens, keeping them from harming the
body.
Vocabulary Review
Immunity is the body’s ability to resist the
pathogens that cause a particular disease.
Vocabulary Review
A vaccine is a preparation of dead or
weakened pathogens that is injected into
the body to cause the immune system to
produce antibodies.