Secondary Immune Response
why do you never normally get the same diseases twice?
In order to kill invading organisms, your body's soldier cells must
first recognize the invaders as bad guys. But this is not easy.
After all, your body contains a large variety of its own cells -- how
can the soldier cells distinguish between a cell from your own body
and a foreigner? If YOU saw a couple thousand of your own cells, of
various types, mixed up with a handful of invader cells, you would be
hard pressed to see the difference.
Because of this difficulty, it takes a little bit of time for the
body to recognize invaders it's never experienced before. I don't
know exactly why -- perhaps an immunologist would. Hence the first
time you are exposed to invaders, your body cannot mount an effective
defense for a little while. You get sick as the invaders multiply and
release various chemical poisons. Finally your army recognizes the
invaders as bad guys and soldier cells specially equipped to kill them
start multiplying. Since the soldier cells have the home field
advantage, they multiply faster, and eventually your army wipes out
the invaders and you get better.
Your army is not stupid. After the invaders are wiped out, a few
specialist cells are kept around who can instantly recognize the same
bad guys, if they turn up again, and sound the alarm. If the invaders
return, the soldiers can start multiplying immediately. Since the bad
guys don't have a head start this time, they get wiped out almost
immediately, and you don't begin to feel sick. (In other words, you
are being infected all the time with the same bugs, but after the
first time you wipe them out so fast that you experience no symptoms.)
It's not enough to recognize invading cells, however. Consider
viruses. A virus is just a loose DNA ``program'' that finds its way
into the ``computer'' (nucleus) of one of your cells. It instructs
the cell to make more viruses instead of doing what it's supposed to
do. How can your army fight this? Well, it must recognize which of
your own cells have been taken over by the enemy and reprogrammed and
kill them. This, again, takes time the first time, and then it
becomes faster. So you become immune to viruses, too.
Viruses are very small and simple, however, and so they can change
easily. Every time they change a little bit, they seem like
completely new invaders to your army, and it must learn to destroy
them all over again. The cold and influenza viruses, for example,
change every year or so. You keep getting infected, sick, and then
immune to them, but there's always a new one next year.
The AIDS virus is particularly fiendish this way. It changes very
fast once it infects a person. The body keeps recognizing the new
variants and wiping them out, but eventually, after many years of
stalemate, the virus finds one particular form that the body can't
recognize, and that's when full-blown AIDS sets in.
What happens if the body makes a mistake and attacks healthy cells
instead of virus-reprogrammed cells? Then you have an ``autoimmune''
disease, such as arthritis, diabetes I, inflammatory bowel disease,
multiple sclerosis, or lupus.
What happens if the body fails to attack cells that have been
reprogrammed by a virus, chemical, or accident? Cancer.
So you see, your army walks a fine line between too hair-trigger of
a response (autoimmune disease) and too lax a watch (cancer).
Incidentally, the invaders do not ``want'' to wipe you out.
Indeed, if they do so, they lose their lives, too. A disease that
kills quickly is, ipso facto, an evolutionary failure, like a dinosaur
that eats up all its food supply and then starves to death. What
happens is that bacteria try to colonize you, to live happily inside
you. But they don't know how. They release waste products that
accidentally poison you, and then they get noticed and wiped out by
your army. Successful bacteria, like E. coli, learn to coexist
peacefully inside you. So disease can be described more accurately as
failed inter-species negotiation than as warfare.
Your have a wonderful immune system that remembers the antigens (foreign
protein identifiers) of diseases. When you are invaded a second time, your
body can respond quickly from its memory cells to prohibit the disease from
becoming infested. You may have felt a cold coming on with a sore throat
starting only to wake up the next day with no symptoms of any kind. This
may have been your immune system quickly dealing with the invasion.
The complexity of this process requires some study on your part if you are
interested. A good high school biology textbook will do the trick in
educating you further.
Anything that enters you body that your immune system doesn't recognize is
called an antigen. Your body has an army of cells whose job it is is to look
for and destroy any antigens that enter. Some of these cells release
proteins called antibodies. When an antigen is seen a specific antibody is
designed especially for it. It takes a while for these antibodies to start
to work and that's why it takes time for you to get better. At the same time
a group of cells is created that is designed to remember that antigen if it
ever comes back. They patrol your body and are on the lookout for that
antigen and if they see it, they snatch it up so fast that your body doesn't
even know it was there. So you don't get the same diseases twice.
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Update: June 2012