An allergy is caused when a person’s immune system attacks a harmless substance, such as pollen, animal dander or peanuts. Because your body thinks the particle is dangerous, it releases chemicals such as histamine to protect itself. The result: red, itchy eyes, sneezing, hives or the potentially life-threatening anaphylactic shock.

The most common allergies are divided into two categories—food and non-food. Popular triggers for the former include milk, egg, peanuts, tree nuts, shellfish, fish, soy, corn and wheat. According to Dr. Sami Bahna, Chief of the Allergy & Immunology section of Louisiana Sate University Health Sciences Centre in Shreveport, Louisiana, food allergy is more common in children than in adults, affecting almost 6 percent of children. And the majority of cases happen before a child reaches her first birthday.

Non-food allergy sufferers are mostly allergic to dust mites, furry pets, mold, and the pollens of grass, trees and weeds. Their symptoms can be easily aggravated by inhaled irritants such as cigarette smoke, fragrance, cleaning solutions and fresh paint.