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. 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.
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