By Charu Kapoor Allergy affects one out of six Americans and thus it becomes important to discuss what actually is allergy, how can it happen, and methods to prevent it. To put it simply, allergy can happen to anyone from substances categorized as allergens. These allergens make the immune system of the body to react and hence the person suffers from allergy. An allergen is a substance which is harmless otherwise, but may cause allergy in the people sensitive to it. Some examples of allergens are pollens, molds, dust, dead skin cells from animals known as animal dander, foods, medication, cockroach droppings, and animal stings.