An allergic disorder is a medical condition in which the body's immune system reacts abnormally to certain substances. Asthma is one of the most common allergic disorders, and is characterized by difficulty breathing, wheezing, coughing, and chest tightness. It is caused by inflammation of the airways, which can be triggered by allergens such as dust, pollen, pet dander, and mold. Treatment for asthma typically involves avoiding triggers, taking medications, and using inhalers.