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En Espanol 
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| Step 3: What Substances Cause Allergies? |
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Allergens are everywhere -- in the air we breathe, in the food we eat, in the medicine and cosmetics we use, and in plants in our backyard. Here are the most common ones.
Airborne allergens
Airborne allergens are the hardest to avoid, particularly pollen. Inhaling an allergen is the most common way people are exposed. This explains why allergy to pollen (also referred to as "hay fever") affects approximately 35 million Americans. Read detailed information on the most common inhaled allergens:
Food
Any food can cause an allergic reaction. However, the foods that most commonly cause food allergy include peanuts, wheat, tree nuts, fish, shellfish, eggs, milk, and soy. Symptoms of food allergy can include stomach cramps, nausea, vomiting, or diarrhea. Some foods, including peanuts, celery, and shellfish, can cause anaphylaxis, a life-threatening condition that requires immediate medical attention. Food allergy is often confused with food intolerance and other conditions, which are not immune system reactions, but may have similar gastrointestinal symptoms. Read more about food allergy.
Insect stings
Insect stings usually cause localized pain and swelling in all people. In people allergic to insect stings, however, they can also cause anaphylaxis. An allergic reaction to insect sting venom sometimes happens with stunning swiftness because the allergen is injected directly into the bloodstream. Read more about insect sting allergic reaction.
Medications
Allergies to medicines may produce symptoms throughout the body. The reactions might be severe, including anaphylaxis. Often, though, these allergies produce symptoms that are quite mild. When medications are injected, the allergic reactions are more likely to appear quickly. Read more about drug allergies.
Latex
Latex and additives used in the processing of latex can cause an allergic reaction. This has become a growing problem, particularly in the healthcare community, due to the increased use of latex as barrier protection against HIV and other contagious diseases. For people allergic to latex, repeated exposure can lead to anaphylaxis. Read more about latex allergy.
Poison ivy, oak, and sumac
Poison ivy, oak, and sumac are found throughout the United States. These plants give off an oil called urushiol. Very small amounts of this oil can cause allergic contact dermatitis, an itchy and blistery rash. This is a classic example of an allergy that does not need antibodies to cause a reaction. Read more about poison plants.
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Reviewed By: Alan Greene, M.D., F.A.A.P., Department of Pediatrics, Packard Children's Hospital, Stanford University School of Medicine; Chief Medical Officer, A.D.A.M., Inc., and Jacqueline A. Hart, M.D., Department of Internal Medicine, Newton-Wellesley Hospital, Boston, Ma.
