BODY DEFENSES
In the past, infectious diseases took a huge
toll. Less than a century ago, every fifth child died before reaching his first
birthday, and almost as many more died before reaching the age of two, most of
them from dysentery and various childhood infectious diseases. Epidemics of diphtheria
have been known to kill every child under 12 in an entire com- munity.
Plagues of smallpox, yellow fever, and other diseases are decimated
populations. Until medicine learned about microorganisms, there was little that
could be done to prevent disease-other than what the body itself could do. And,
fortunately, the body is not helpless against germs; else man would have ceased
to exist at all long ago. Against an overwhelming influx, body defenses, as
already noted, may not suffice. But those defenses, day in and day out, serve
to prevent many instances of what otherwise could be serious disease. There are
actually several lines of defense, and it is important to under- stand all of
them. The first line prevents organisms from gaining entrance. The skin itself
is part of this vital defense.
Unless broken by a wound, it is practically microbe-proof.
Organisms lie harmlessly on it. Body openings also are defended. Mucous
membranes form a protective lining. In the respiratory passages, the membranes'
secretions of mucus form a sticky coating that catches many organisms before
they can penetrate farther. Tiny filters, such as the hair in the nose, help
keep out germs. Irritation of the mucous membranes leads to coughing and
sneezing, which force out germs and irritating substances. Body fluids have
defense functions.
Tears wash organisms out of the eyes and, being slightly
antiseptic, discourage organism growth. Stomach acid kills many organisms
entering with food. Vomiting and diarrhea are part of the defense mechanism,
helping to get rid of invading organisms. A second line of defense goes into
action when organisms get by the first line. It consists of leukocytes (white
blood cells) and tissue cells. As organisms and their poisonous products begin
to damage tissue cells, the body increases the blood supply to the infected
area, producing an inflammatory reaction. And as cells are destroyed, others
form a wall around the invading organisms, aimed at confining them and
protecting other body areas. It is this wall that surrounds a boil, infected
cut, even a pimple.
Fever, too, develops in many infections and sometimes helps
in the destruction of some germs that are unable to thrive at a temperature
much above normal body temperature. Meanwhile, the white blood cells are at
work. The number in the blood increases rapidly when they are needed to fight
infections. Moving into an infected area, the white cells attack and devour the
organisms. The infected area becomes a mass of living and dead organisms and
white cells, forming the pus that develops in infected tissue. Lymph helps
clear out the debris, moving it to the lymph nodes where it is filtered and
gradually destroyed.
This is why swollen lymph nodes are indicators of infection
in the body. Thus, often, infection is conquered by second-line defense in the
tissues, sparing the body more general combat. But if the infection prevails,
other body resources are available. The body produces materials called
antibodies, which can counteract germs and render their poisons harmless. There
are varioustypes of antibodies. One, antitoxin, is highly specific, acting
against only a toxin formed by a particular kind of organism. For example,
diphtheria anti- toxin neutralizes diphtheria toxin but not scarlet fever or
any other kind. Other antibodies, called agglutinins, make certain types of
organisms clump together, turning them into easier targets for white blood
cells, which surround and devour them. Still other antibodies, bacteriolysins,
make certain types of organisms dissolve in the bloodstream.
After the body has won its battle against some types of
disease, anti- bodies remain in the blood and prevent the organisms of those
particular diseases from getting footholds again. This is what happens when you
have measles, chickenpox, or any of the other diseases that people do not get
more than once. In such cases, the body becomes immune to the disease; this is
called acquired immunity. Many of us are immune to one or more forms of polio
because we have had an attack that was so mild we didn't even know we had it,
usually mistaking it for flu. Some types of immunity can be passed from mother
to child. People, who are immune to a disease withoutever having had are said
to have natural immunity.
Some immunityis partial or temporary-that is, they do not
completely protect an individual an entire lifetime (a notable example is the
common cold).
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