Science High School Reviewer | Grade 4 Science | Life Science | Systems of the Human Body | Lesson 15: How does the body defend itself?

The human body has many ways to prevent
disease-causing organisms from getting into tissues.
Microorganisms in Your Body
When you get a cut or scrape, you should clean it and
cover it with a bandage so the wound will not get infected.
You get an infection when disease-causing organisms enter,
live in, and multiply within your body. Most organisms that
cause diseases are so small that they can be seen only with a
microscope. That’s why they are called microorganisms.
Not all microorganisms that live in your body cause
disease. In fact, many microorganisms are in your body all
the time and cause no problems. Microorganisms are on
your skin, in your mouth, and in your digestive system. Most
of them are harmless as long as they stay where they belong.
Your Body’s Defenses
Your body uses special cells, tissues, organs, and
chemicals to keep disease-causing microorganisms
from causing harm. Your skin, breathing passages,
mouth, and stomach are just some of your body’s
defenses against invading microorganisms.
Your skin is your body’s first defense. Your skin
is more than a layer of physical protection. It also
provides chemical protection. Acids in your sweat kill
many microorganisms that can cause disease.
Your body has other special means of protection.
For example, your tears wash away disease-causing
microorganisms that touch your eyes. Your tears
contain chemicals that kill certain microorganisms.
In your mouth, mucus and saliva can trap and
then wash away microorganisms that can
cause disease.

(diagram Scott Foresman Gr4)

If you swallow
disease-causing
organisms, acids in
your stomach will
kill many of them.

Thick, slippery mucus in
your throat, nose, and
lungs traps dust and many
microorganisms you breathe
in. Coughing or swallowing
removes the mucus and
microorganisms.

Your skin is made
of tightly packed
cells that cover
your body. The
column of cells
colored green is
a hair.

Bacteria and Viruses
Organisms that cause some diseases are
called pathogens. Pathogens do not belong
in your body. If they enter your body and
multiply, they create an infection and you
develop a disease.
Diseases caused by pathogens are infectious.
An infectious disease is a disease that can
pass from one organism to another. When you
have an infectious disease, pathogens have
gotten inside your body and harmed it. If
you have an infectious disease, you can
pass it to someone else.

Bacteria and viruses are pathogens
that cause infectious diseases. Viruses
are about 100 times smaller than
most bacteria. Viruses are not made
of cells and have no nucleus. They
can reproduce only by using living
cells to create more viruses. Different
types of viruses invade different types
of cells. For example, viruses that
invade the cells of your nose, mouth,
or throat can give you a cold.

Staying Healthy
Some microorganisms travel through
the air. So, if you are sick, cover your mouth
and nose when you sneeze or cough. Many
diseases are spread by direct contact between
two people or between a person and an object.
You can do a few simple things to reduce your
chance of catching diseases by direct contact.
Wash your hands before you eat. Make sure
that utensils and objects have been properly
cleaned before you use them. Objects such as
towels, drinking glasses, and silverware can
pass disease-causing microorganisms from one
person to another.

Sometimes people are
infected by food that carries
harmful bacteria. Many
cases of food poisoning are
caused by harmful strains of
bacteria known as E. coli.

All these photographs
were taken through special
microscopes that magnified
these images many
thousands of times.

Attacking the Invaders
Microorganisms called pathogens can cause disease if
they get past your body’s defenses. Your immune system
helps protect you from many pathogens. Your immune
system is made of blood cells and other tissues.
Special white blood cells join together to destroy
pathogens that have begun to damage other cells. These
blood cells also make antibodies that attach to pathogens.
An antibody is a chemical that the body makes to stop
pathogens from infecting other cells. A different antibody
is made for each kind of pathogen.
Your immune system uses antibodies that your body
already has. If you have antibodies to a pathogen, your
immune system will attack whenever that pathogen enters
your body. If the antibodies prevent you from getting sick,
you are immune to the disease.
Preventing the Disease
A vaccine is a kind of medicine that protects you from
a disease. The vaccine signals your immune system to make
antibodies to a certain pathogen. You develop immunity
without ever having the disease.
Vaccines are not available for all infectious diseases. You
can help protect yourself by eating healthful foods, getting
plenty of rest and exercise, and washing your hands often.
Sometimes a person’s own immune system produces
antibodies that attack a pathogen very quickly. For example,
once you have chicken pox, you will probably never get
it again. The first time you have chicken pox, your body
produces antibodies that attack the disease. The next time the
chicken pox virus enters your body, the antibodies will attack
it before you ever become sick.