AUTOIMMUNE DISEASES 185The "recruitment center" for the system is in the marrow of our
bones. The marrow is responsible for generating specialized cells
called stem cells. Some of these cells are released into circulation for
use elsewhere in the body; these are called B-cells (for bone). Other
cells formed in the bone marrow remain immature, or unspecialized,
until they travel to the thymus (an organ in the chest cavity just above
the heart) where they become specialized; these are called T-cells (for
thymus). These "soldier" cells, along with other specialized cells, team
up to create intricate defense plans. They meet at major intersections
around the body, including the spleen (just inside the left lower rib
cage) and the lymph nodes. These meeting points are like command
and control centers, where the "soldier cells" rearrange themselves into
teams to attack foreign invaders.
These cells are remarkably adaptable when they form their teams.
They are able to respond to different circumstances and different for-
eign substances, even those they have never before seen. The immune
response to these strangers is an incredibly creative process. It is one of
the true wonders of nature.
The foreign invaders are protein molecules called antigens. These
foreign cells can be a bacterium or a virus looking to corrupt the body's
integrity So when our immune system notices these foreign cells, or
antigens, it destroys them. Each of these foreign antigens has a sepa-
rate identity, which is determined by the sequence of amino acids that
comprises its proteins. It is analogous to each and every person having
a different face. Because numerous amino acids are available for creating
proteins, there are infinite varieties of distinctive "faces."
To counter these antigens, our immune system must customize its
defense to each attack. It does this by creating a "mirror image" pro-
tein for each attacker. The mirror image is able to fit perfectly onto the
antigen and destroy it. Essentially, the immune system creates a mold
for each face it encounters. Every time it sees that face after the initial
encounter, it uses the custom-made mold to "capture" the invader and
destroy it. The mold may be a B-cell antibody or a T-cell-based receptor
protein.
Remembering each defense against each invader is what immuniza-
tion is all about. An initial exposure to chicken pox, for example, is a
difficult battle, but the second time you encounter that virus you will
know exactly how to deal with it, and the war will be shorter, less pain-
ful and much more successful. You may not even get sick.