For the immune system,
life is hard. It is a 24-hours-a-day, 7-days-a-week,
52-weeks-a-year battle against a well-equipped
and persistent army trying to harm your health.
The immune system never rests and must always be
on red alert. It takes no furloughs.
who make up the immune system come from and are
found in a diverse collection of organs. Although
the components, when taken together, weigh only
about two pounds (900 g), these two pounds are
integral in keeping the scale of good health
and most easily seen component of the immune
system is the skin. The skin is a physical
barrier against pathogensharmful bacteria,
viruses, and fungiand also a chemical
barrier: the skins natural acidity is a
poor environment for invaders.
The mucous membranes are the
sentries at the gates of our body: the openings
of the eyes, sinuses, mouth, and so on. They
secrete mucus, which both cleans away and traps
pathogens. If bacteria should progress through
the mouth or nose and into the stomach, it is
still difficult for them to get into the blood.
They must survive the stomach, which to them is a
poisonous torture chamber of acids and digestive
pathogen breaches a gate and enters the body,
other components of the immune systemwhite
blood cellsgo to work. One type of white
blood cell, a phagocyte, is like the skin in that
it counters all invaders. Other types of white
blood cells, in the class known as lymphocytes,
are programmed to go after only certain pathogens.
The various types of white blood cells all work
in different ways, but they all need each other
to complete the job of protecting the body.
eaters") are large white blood cells that
engulf and digest pathogens. An important type of
phagocyte is monocytes, which circulate
throughout the body looking for troublelooking
for pathogens. When monocytes get into tissue,
they develop into macrophages ("big eaters").
Macrophages are positioned throughout body tissue
and are often specialized: you might say some
have an appetite for pathogens found in the lungs,
while others prefer the taste of those found in
the kidneys. Macrophages also are the "sweepers"
of the body, as they dispose of worn out cells.
phagocytes are granulocytes. Of these, mast cells
are found in the tissue, and neutrophils,
eosinophils, and basophils are found in the blood.
Lymphocytes are small white
blood cells that travel through the lymph system.
They are not able to attack just any pathogen, as
phagocytes are. They are programmed to go after
specific pathogens. They also bear the major
responsibility for the actions of the immune
two major classes of lymphocytes are B cells,
which reach maturity in the bone, and T
cells, which reach maturity in the thymus.
Both of these recognize specific pathogens.
cells work by producing, transporting, and
secreting antibodies. Upon meeting a pathogen, B
cells begin dividing and releasing antibodies,
which seek out and destroy the pathogen. Immune
system components known as "complements"
also aid the antibodies in destroying pathogens.
B cell makes one specific antibody for one
specific pathogen. If a B cell meets another
pathogen, nothing can be done. One type of T cell,
a helper T cell, must be involved for a B cell to
cells can help B cells or other T cells, or
directly attack pathogens. When they directly
attack the pathogen, they are known as cytotoxic
T cells. Another important T cell is the natural
killer (NK). These T cells are similar to
phagocytes in that they do not need to recognize
a pathogen to swing into action. They are
important in targeting tumor cells.
are also suppressor T cells, which act as the
referee in the battle. When the immune system has
won a battle, the suppressor T cells call off the
Bone marrow: The
soft tissue in the center of bones cells,
including white blood cells.
Thymus: The thymus
fosters development of T cells.
Lymphatic vessels: The
lymphatic vessels are arteries that carry
white blood cells throughout the body.
Lymphatic nodes and the
spleen: The nodes and spleen are
both "stopping pints" for white