Transcript The Body`s Defenses

The Body’s Defenses
Barriers To Infection:Our First Line
of Defense- Stopping the invasion
 The body has main barriers to infection that may
stop pathogens from ever getting into the body,
causing infection. These are the Skin, Breathing
Passages, the Mouth/Stomach.
 1.) Skin: Acts like a barrier several ways. First,
chemicals in the oils and sweat on your skin kill many
pathogens when they land on it. Second, as the surface
layer of dead skin cells flake off they take many
pathogens with them. Finally, the epidermis’s tough
layers of dead cells acts like a shield keeping many
pathogens out.
Barriers (cont.)
2.) Breathing Passages: These
act as barriers in two ways.
 First, mucus traps pathogens and cilia (little
hair-like structures) sweep away most
pathogens trying to enter.

Second, by coughing or sneezing we expel
many pathogens.
Barriers (cont.)
3a.) The Mouth:
saliva contains
chemicals that kill some pathogens.
3b.) The Stomach:
Many of the
pathogens that survive your saliva wind
up in your stomach when you swallow
and are destroyed by your stomach
acids.
The Inflammatory Response:Our
Second Line of Defense
 Ok, germs got past our first line of defense and inside
us. Now What?
 When body cells are damaged by invading
germs they release chemicals. These
chemicals trigger our second line of defenseThe Inflammatory Response to kick in.
 First, blood vessels enlarge releasing special white
blood cells into affected tissues that get involved
during the inflammatory response called phagocytes
which engulf and destroy pathogens.
 Second, these chemicals also may cause a fever,
which often stops the growth and reproduction of
pathogens, allowing phagocytes to clear them from the
The Immune Response:Our Third
Line of Defense
 If an infection is severe enough to cause a
fever, our third line of defense is triggeredThe Immune Response.
 The special white blood cells of the Immune
system can tell the difference between
different types of pathogens and develop a
defense specifically targeting that pathogen.
 The Immune system’s special white blood
cells are called Lymphocytes.
The Immune Response (cont.)
 There are two different types of Lymphocytes; T
lymphocytes (or T cells) and B lymphocytes (B
cells). They each have different jobs but work
together to destroy pathogens.
 We all have tens of millions of T cells circulating
in our bloodstream. When a T cell encounters
a pathogen it identifies it based on unique
marker molecules that each type of pathogen
has on its surface. These marker molecules
are called Antigens.
The Immune Response (cont.)
 All cells have antigens. Each person has
identical antigens which tell a T cell whether
a cell it encounters is yours or an invader.
 When T cells identify an invader they activate
B cells to produce Antibodies for that
pathogen.
 . Antibodies are chemicals that lock onto a
pathogen’s antigens and mark them (like
signs) for destruction by phagocytes.
AIDS: A Disease of the
Immune
System
 Is the only disease where viruses attack
the immune system directly.
 Is caused by the HIV (Human
Immunodeficiency Virus).
 It invades and destroys T cells. Without
their protection, you have no protection
against other invading pathogens,
resulting in illness and later death.
AIDS: (continued)
 It is spread only through the transmission
of body fluids (blood or human
reproductive fluids) from an infected
person to an uninfected person.
 This can occur as a result of sex, infected
mothers passing the virus to her infant in
the womb or when nursing, or through the
sharing of needles by intravenous drug
users.