An allergy is a reaction of a person's immune system to substances that are usually harmless and don't bother most other people. Common substances that cause allergies, also known as allergens, are : dust, pollen, pet dander, mold spores, certain food items and medicines. Usually allergic reactions can cause : hay fever, eczema, hives, asthma, runny nose, itchiness, rashes, swelling and other skin reactions. In some cases, allergies to certain medicines can cause serious life-threatening reactions.