Allergic asthma is the most common form of asthma. Mostly the symptoms of allergic and non-allergic asthma are the same. The allergic asthma is triggered by inhaling allergens. An allergen is a harmless substance such as dust mites, pet dander, pollen or mold. If a person is allergic to a substance, this allergen triggers a response starting in the immune system. Through a complex reaction, these allergens then cause the passages in the airways of the lungs to become inflamed and swollen. This results in coughing, wheezing and other asthma symptoms.