|
What
is Asthma?
In many
people, asthma appears to be an allergic
reaction to substances commonly breathed in through the air,
such as animal dander, pollen, or dust mite and cockroach
waste products. The catch-all name for these substances, allergens,
refers to anything that provokes an allergic reaction. Some
people have a genetic predisposition to react to certain allergens.
When
these people breathe in the allergen, the immune system goes
into high gear as if fighting off a harmful parasite. The
system produces a molecule called immunoglobulin E (IgE),
one of a class of defensive molecules termed antibodies. The
IgE antibody is central to the allergic reaction. For example,
it causes mast cells, a type of specialized defensive cell,
to release chemical "weapons" into the airways. The airways
then become inflamed and constricted, leading to coughing,
wheezing, and difficulty breathing -- an asthma
attack.
Without
treatment, such as inhaled corticosteriods to reduce the inflammation,
asthma attacks can be deadly. The overall
death rate for asthma, however, is low.
Why
is Asthma on the Rise?
Although
several theories exist about why asthma rates
have risen during the last two decades, there probably is
no simple answer, says Calman Prussin, M.D., head of the clinical
allergy and immunology unit at NIAID.
One theory
is that people today, especially in developed countries, are
spending more time indoors, Dr. Prussin says. We are therefore
exposed to more indoor allergens, such as dust mite allergen,
that cause asthma. "Our houses are now hermetically
sealed to save heating and cooling energy," he notes, "and
unfortunately this causes more indoor allergen exposure."
Another
reason may be that people today live in cleaner, more sanitary
conditions than they did before the industrial revolution,
relatively free of disease-causing viruses and bacteria, he
says. This clean living affects our immune system. The immune
system's defensive white blood cells, called T cells, have
two basic "settings," he explains. Th1 cells fight infectious
viruses and bacteria. Th2 cells fight parasites but are also
involved in allergic reactions.
"We are
exposed to fewer viruses and bacteria than people were 100
years ago, so perhaps our immune systems have not learned
to make Th1 cells as well," Dr. Prussin says. "That means
we have a greater proportion of Th2 cells in our bodies, which
might lead to more allergies and asthma."
Other
theories point to increased levels of air pollutants, a decline
in the amount of exercise people get, or rising obesity as
factors in the increase of asthma.
Common typos:
astmha, ashtma, atshma, sathma, asthm, astha, astma, ashma,
athma, sthma, asthna, astham,
| niaid.nih.gov
|
|
