In asthma, the airways are sensitive to triggers such as dust mites, cigarette smoke, pollen grains and stress.The cells lining the bronchioles release histamines, which make the epithelial cells become inflamed and swollen.Goblet cells make excess mucus and the smooth muscles in the walls of the bronchioles contract.The airways narrow making it more difficult to breathe.Relievers such as chemicals similar to adrenaline and preventers such as steroids reduce the symptoms.