Introduction |
The immune system is a complex network of specialized cells and
organs that has evolved to defend the body against attacks by
ìforeignî invaders. When functioning properly it
fights off infections by agents such as bacteria, viruses, fungi,
and parasites. When it malfunctions, however, it can unleash
a torrent of diseases, from allergy to arthritis to cancer to
AIDS.
The immune system evolved because we live in a sea of microbes.
Like man, these organisms are programmed to perpetuate themselves.
The human body provides an ideal habitat for many of them and
they try to break in: because the presence of these organisms
is often harmful, the bodyís immune system will attempt
to bar their entry or, failing that, to seek out and destroy
them.
The immune system, which equals in complexity the intricacies
of the brain and nervous system, displays several remarkable
characteristics, It can distinguish between "self"
and ìnonself.î It is able to remember previous experiences
and react accordingly: once you have had chicken pox, your immune
system will prevent you from getting it again. The immune system
displays both enormous diversity and extraordinary specificity:
not only is it able to recognize many millions of distinctive
nonself molecules, it can produce molecules and cells to match
up with and counteract each one of them. And it has at its command
a sophisticated array of weapons
The success of this system in defending the body relies on an
incredibly elaborate and dynamic regulatory-communications network.
Millions and millions of cells, organized into sets and subsets,
pass information back and forth like clouds of bees swarming
around a hive. The result is a sensitive system of checks and
balances that produces an immune response that is prompt, appropriate,
effective, and self-limiting.
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| Self and Nonself |
At the heart of the immune system is the ability to distinguish
between self and nonself Virtually every body cell carries distinctive
molecules that identify it as self.
The bodyís immune defenses do not normally attack tissues
that carry a self marker. Rather, immune cells and other body
cells coexist peaceably in a state known as self~tolerance. But
when immune defenders encounter cells or organisms carrying molecules
that say "foreign" the immune troops move quickly to
eliminate the intruders.
Any substance capable of triggering an immune response is called
an antigen. An antigen can be a virus, a bacterium, a fungus,
or a parasite, or even a portion or product of one of these organisms.
Tissues or cells from another individual except an identical
twin whose cells carry identical self-markers also act as antigens,
because the immune system recognizes transplanted tissues as
foreign, it rejects them. The body will even reject nourishing
proteins unless they are first broken by the digestive system
into their primary, non-antigenic building blocks.
An antigen announces its foreignness by means of intricate and
characteristic shapes called epitopes, which protrude from its
surface. Most antigens, even the simplest microbes, carry
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