An allergy is an abnormally high sensitivity to an otherwise harmless foreign substance, or allergen, that leads to an exaggerated response of the natural immune defense mechanism. Allergens can be airborne substances, infectious agents, foods, contactants, or physical agents; and reactions can range from the relatively benign runny nose to severe inflammation, anaphylactic shock, and even death. Why do some scientists believe that children with no siblings are more prone to allergies? Discuss