Asthma, also known as reactive airway disease, is a chronic lung condition with:

• Inflammation (swelling) of the airways
• Increased sensitivity of the airways to a variety of things that make asthma worse
• Obstruction of airflow

Inflammation. Inflammation of the lining of the airways is the most common feature of asthma. When they are stimulated, certain cells lining the airways release chemical substances (mediators) that lead to inflammation. This causes the airway lining to swell and narrow. The inflammation may last for weeks following an episode. Most people with asthma have some degree of inflammation all the time. Some long-term-control medications can help prevent and reduce inflammation.

Increased sensitivity. Another characteristic of asthma is increased sensitivity of the airways. When inflammation occurs in the airways, the airways become more sensitive. When the airways are more sensitive, you are more likely to have asthma symptoms when exposed to things that make asthma worse. When there is less inflammation, the airways are less sensitive and you are less likely to have asthma symptoms when exposed to things that make asthma worse.

Airway obstruction. In addition to inflammation, further airway obstruction occurs with asthma. Obstruction is caused by tightening of muscles that surround the airways. This is also called bronchospasm. Bronchospasm causes further narrowing of the inflamed airways. In some people with asthma, the mucous glands in the airways produce excessive, thick mucus, further obstructing the airways.