An allergy is an unusually strong reaction to a substance. Many things can cause allergies. The most common cause is pollen. Other causes include organisms such as dust mites and molds. Chemicals, plants and dead skin particles from dogs and cats can also cause allergic reactions. So can insect stings and some foods.The most common kind of allergic reaction is itchy, watery eyes and a blocked or watery nose. Allergies can also cause red, itchy skin. Some reactions can be life-threatening -- for example, when breathing passages become blocked.