The human body is a fascinating “system of systems.”
All of these systems must work together to keep the
body operating properly. Most of the time they work
well and the body functions as it should. There are,
however, environmental conditions and other
organisms that can negatively affect how the body
functions. Micro-organisms, such as bacteria, that
cause diseases are referred to as pathogens.
Pathogen = any micro-organism that causes a disease
If pathogens are able to enter the body, they either
interfere directly with cells or tissues, produce
toxins (poisonous chemicals) that can affect the
normal functioning of the body. Fortunately, the body
has developed a number of ways to defend itself
from invasion by foreign organisms and their toxins.
The body’s first line of defence is the skin
• its function is to keep foreign invaders outside the
body
• it cannot normally be penetrated by bacteria or
viruses unless it is broken
• the skin produces natural acidic oils and sweat that
prevents micro-organisms from growing
That the skin is the largest organ of the body with a
surface area of about 2 m2. The top layer of skin
cells is constantly being replaced by new cells, so you
have an entirely new skin about every month!
Where the skin leads to the inside of the body, such
as in the mouth, nose, eye, and ears, the body has
other physical barriers that prevents invaders from
entering.
1. Hair
• tiny hairs filter out dust, dirt, and micro-organisms
that are in the air
2. Ear Wax
• waxy material that traps any foreign particles and
micro-organisms that enter
3. Mucus
• Found in the mouth, nose, and throat
• Sticky substance that acts like flypaper
• Removed by coughing or swallowing
4.Tears
• Contain a special chemical that kills bacteria
5. Gastric Juices
• Micro-organisms that sneak past the throat will be
destroyed by the strong acid in the stomach
• Men have more nose hair than women
• That not all people’s ear wax is the same. Ear wax
can be gray, yellow, pumpkin-coloured, or brown. It
can also be moist or dry
• In a year, you are likely to shed about 4 L of tears
• When you sneeze, the air (and mucus) inside your
nose hurtles out at a speed of over 100km/hr
• You recycle about 1 L of mucus per day by
swallowing
The body’s second line of defence are the white
blood cells. When invading organisms enter the body
through a break in the skin, special white blood cells
move from the bloodstream to the injured area.
These white blood cells detect, capture, and destroy
invading organisms just like lysosomes.
Lets take a look at how white blood cells work…
1. Invaders release a
chemical that
signals the
Secondary
Defence System
2. This chemical
signal causes
increased blood
flow to the
injured area and
attracts special
white blood cells
3. The white blood
cells engulf and
digest the
invaders
4. The body can then
heal the tissues in
the injured area
The remaining fragments of dead white blood cells
and digested invaders are called pus. The presence of
pus is a sure sign that the second line of defence is
working.
Although the two lines of defence are generally
successful in protecting the body, they are not
always able to stop the invasion. For example, microorganisms can enter the body in food or water (e.g. E.
Coli). They can also be transmitted by insect bites
(e.g. Malaria & West Nile virus). When a pathogen
does get past the first two lines of defence, the
body’s tertiary defence system (the Immune System)
is called into action.
• all foreign organisms produce a chemical called an
antigen
• this antigen causes the production of a specific
antibody that attaches only to this antigen
• toxins attach to receptor
sites on cells and interfere
with normal functions
• antibodies also attack
toxins preventing them from
attaching to the cell
The use of antibodies to fight a pathogen is called an
immune response. In many cases, the first exposure
to a pathogen causes the body to produce its own
antibodies, which protect the body against any future
attacks by the same pathogen. In other cases,
temporary or permanent immunity is achieved by
vaccination.
Vaccine = an old or weakened version of a pathogen