Asthma is a chronic lung condition that makes it difficult to breathe. People with asthma have airways that are very sensitive and easily irritated. When those airways become inflamed, they narrow so that the air can't move in and out as easily. Symptoms of asthma include wheezing, coughing, tightness in the chest, and shortness of breath. Each of these can aggravate the other symptoms, so the sufferer is caught in a vicious cycle. And if the airways constrict too much, oxygen cannot reach vital organs. People can die from severe asthma attacks.
Asthma is not technically an allergy, but common allergens are most likely to trigger an attack. Inflammation is the body's response to foreign or toxic substances that come into contact with the body. Allergens such as pollen, house dust mites and animal dander can all trigger an immune system reaction in the airways. Just like a bacterial infection, this signals the body to bring white blood cells to the irritated area, and these attack the allergens.
In the process, the tissues of the bronchial tubes can become damaged. So the tubes become hyper-sensitive and over-react to things that would not previously have triggered a reaction: cold weather, pollution, dry air, or chemical odors, for instance. The bronchial tubes can also go into spasms and narrow, causing wheezing. Mucus can't be as easily cleared and accumulates in the bronchial tubes, setting off a cough reflex to clear the extra mucus out of airways.