Asthma is an allergic reaction to substances breathed in through the air, such as animal dander pollen or dust.

When people who are allergic to a substance breathe it in, the immune system treats it as a harmful invader and produces a molecule called immunoglobulin E (IgE), an antibody to fight the substance. This, in tum, can cause airways to become inflamed and constricted, leading to coughing and wheezing as well as difficulty breathing.

Approximately 17 million Americans, or 6.4% of the population, has asthma, according to the National Institute of Allergy and Infectious Diseases.

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