Part
One: Introduction
The immune system
is a complex network of specialized cells and organs that has
evolved to defend the body against attacks by "foreign" 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 body's 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 "nonself." It is able to
remember previous experiences and react accordingly; thus, 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.
