Defenses Against Disease
Communicable Diseases
If pathogens are everywhere, why
aren’t you sick all of the time?
When you do get sick, what keeps pathogens
from multiplying until they take over your body?
The Answer
• Your body has a number of defenses against
infection.
• Your body’s first line of defense against infectious
disease includes both physical and chemical
defenses that prevent pathogens from entering
your body.
Skin
• Your skin serves as both a physical and
chemical barrier against pathogens.
• The surface cells are hard and have no gaps
between them.
• Sweat acts as a chemical barrier because it
contains acids that kill many bacteria.
• Old skin cells are shed constantly, and the
pathogens on these cells are shed too.
• In fact, microorganisms usually cannot get
through your skin unless you have a cut,
scrape, burn, or other injury.
Mucous Membranes
Openings to your body, such as your
are covered by protective linings call mucous
membranes. Mucous membranes secrete a fluid
called mucous that traps pathogens and washes
them away. Mucous also contains chemicals and
specialized cells that attack pathogens.
Cilia
• Some of your body’s
mucous membranes
are lined with tiny
hair-like structures
called cilia.
• Cilia and mucous
help trap and remove
pathogens.
• When you cough,
sneeze, or blow your
nose, the pathogens
are removed along
with the mucous.
Saliva & Tears
• Your saliva and tears can trap pathogens and
wash them away. Like mucous, saliva and
tears also contain chemicals that attack
pathogens.
Digestive System
Chemicals in
your
digestive
system,
including
your
stomach, kill
many
pathogens.
In addition, the normal motions of the digestive
system not only move food through your system
but also move pathogens out.
Inflammation
• If pathogens are able to get past the physical
and chemical defenses and to injure cells, your
body is ready with a second line of defenseInflammation.
• Inflammation is your body’s general response
to all kinds of injury, from cuts and scrapes to
internal damage.
• Inflammation fights infection and promotes
the healing process.
Phagocytes
• Within seconds after your body is injured, the damaged cells
release chemicals that cause blood vessels in the injured area
to enlarge.
• Blood, other fluids, and white blood cells called phagocytes
leak out of the enlarged vessels.
The phagocytes engulf and destroy
pathogens.
Meanwhile the
infected area
becomes red,
swollen, and sorein other words,
inflamed.
Healing
• Phagocytes give off substances that cause
healing to begin. The fluids, phagocytes, and
dead cells that accumulate at the injury cite
often result in the formation of a thick, white
liquid called pus.
• Your body’s third line of defense against
pathogens is your immune system.
• The immune system fights disease by
producing a separate set of weapons for each
kind of pathogen it encounters.
When a pathogen enters your body for
the first time, it often causes disease.
If your immune system is working, why does this
happen?
The Answer
• Your immune system must build up its arsenal
of weapons against the newly encountered
pathogen.
• When a pathogen enters your body it takes
time for it to multiply and cause disease.
• Once the immune system’s arsenal is built up,
however, the immune system kills the
pathogen, and your body gradually recovers.
White blood cells called lymphocytes carry
out most of the immune system’s
functions.
Immunity
• If your body has previously been attacked by a
pathogen it will recognize it when it is exposed
to it again.
• This time, your immune system will quickly
recognize the pathogen and launch an
immediate attack!
• When this happens you are said to be
immune.
• Immunity is your body’s ability to destroy
pathogens that it has previously encountered
before the pathogens are able to cause
disease.
Killer T Cells
• Killer T Cells destroy any body cell that has
been infected by a pathogen.
Helper T Cell
• Helper T Cells produce chemicals that
stimulate other T cells and B cells to fight off
infection.
Suppressor T Cells
• Suppressor T cells produce chemicals that
“turn off” other immune system cells when an
infection has been brought under control.
B Cells
• B Cells produce antibodies. Antibodies are
proteins that attach to the surface of
pathogens or to the toxins produced by
pathogens.
This binding action keeps the pathogen or toxin
from harming the body.
Once the
infection is
overcome,
your B cells
stop
producing
antibodies,
but they do
not “forget”
how to
produce
them.
See Figure 5 for The Immune Response
Passive vs Active Immunity
• There are two types of immunity• 1. Passive
• 2. Active
• Both types are important in protecting your
body against disease.
Passive Immunity
• Immunity that is acquired by receiving
antibodies from a source other than one’s
own body.
• This type of immunity is temporary and not
lifelong.
Examples
• It occurs naturally in babies who receive
antibodies from their mothers before birth.
• After birth antibodies may also be passed to
an infant through the mother’s breast milk.
Or
• When a doctor gives someone injections of
rabies antibodies if they were bitten by a dog
with rabies.
Active Immunity
• Active immunity is immunity that your own
immune system creates.
• Active immunity results from either having a
disease or from receiving a vaccine.
Vaccines
• Vaccines contain small amounts of dead or
modified pathogens or their toxins.
• A vaccine causes your immune system to
produce antibodies against a pathogen, as if
you had actually been infected.
• You develop immunity without having to
experience the disease.
Booster Shots
• After a few years, you may receive a booster
dose of some vaccines to “remind” your
immune system to maintain your immunity.