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The immune system involves an
intricate network of molecules and specialized cells that work in
close cooperation to fight infection. We rely upon our immune system
to defend us from attack by foreign invaders, such as bacteria or
viruses.
For our
bodies, the first line of defense against invasion by germs is made
up of the skin and the mucous membranes (for example, those that line
the nose and the throat). Our skin acts as an important physical
barrier. Many of the billions of germs that we have on our skin
surfaces are eliminated when they are shed with the outer layers of
skin.
The mucous membranes are not as tough
as skin and are more vulnerable. However, they contain many natural
substances that fight germs. One such substance, called lysozyme, is
found in tears, saliva, and sweat. While the mere acidity of sweat is
sufficient to impede the growth of many germs, lysozyme kills them by
destroying their cell walls. For that reason, an animal can help heal
its wounds simply by licking them.
If a bacteria capable of causing
disease does break through the protective barrier formed by the skin
and mucous membranes, an army of cells immediately goes into action,
with but one purpose—elimination of the invading germ and
consequent recovery from illness. The cells that fight to defend the
body are called leukocytes, or white blood cells. Three important
types of white blood cells in this stage of the struggle are
monocytes, neutrophils, and lymphocytes. Monocytes penetrate stricken
tissue and devour invading bacteria. Neutrophils help swallow
bacteria and are expelled from the body as pus. Lymphocytes have
immunologic memory; if the same type of germ reappears, they will
produce substances called antibodies, which bind specifically to a
particular germ fragment. There are two principal teams of
lymphocytes with differing abilities:
B cells release
antibodies like well-aimed arrows; these “seek” germs and attack
them
T cells help
produce antibodies, which engage in “hand-to-hand” combat with
germs
—Helper T cells help B cells
secrete large quantities of antibodies
—Natural
killer cells kill infected cells directly without
producing antibodies.
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