What causes allergies?

Particular ingredients or chemicals within your environment, the foods you eat, or the drugs you take can cause allergies. Normally, these substances are harmless. In some people, however, their immune system overreacts and mistakenly attacks these elements. This produces an allergic reaction.

Picture of allergens
Dr. Mehmet Oz, MD
Cardiology (Cardiovascular Disease)

Dr. Oz answers actress Allison Sweeney's question about allergies. Find out what causes allergies and see a painless treatment that everyone can try at home in this video.

Anthony L. Komaroff, MD
Internal Medicine
Because not everyone suffers from allergies, scientists suspect there's a genetic component to allergic reactions that makes some people predisposed to them. For instance, a child of a parent who has allergies has a 50% risk of developing allergies. And this risk increases to 75% if both the child's parents are allergy sufferers.

Other factors also play an important role. For example, some scientists are investigating what is known as the "hygiene hypothesis." Because modern societies have better hygiene than ever before -- in the form of clean water, sanitary toilets, and less exposure to dirt -- people are exposed to fewer microbes early in life, when the human immune system is developing. The hygiene hypothesis holds that this lack of early exposure permanently influences how the immune system responds later in life. For example, T cells may develop differently in children who live in today's cleaner homes than they did before good sanitation was widespread. One possible result is allergies, as immune cells respond to minor irritants like pollen and dander that aren't actually dangerous. So the die may be cast in early childhood, a crucial time for the development of the immune response.
An allergy is when your body overreacts to something that's harmless to most other people. Here are some common things people are allergic to:
  • Pollen and mold. Many people are allergic to pollen from trees, weeds, and grasses. Mold growth is a big problem in wet areas.
  • Dust mites. Dust mites are tiny bugs you can't see but are everywhere. They feed off dust, dirt, dead skin, and other harmless things in our homes.
  • Animals. Many people are allergic to cats, dogs, and other pets. Being around these animals brings on symptoms.
  • Insect bites or stings. If you are allergic, your reaction could affect your whole body. It will be worse than the usual redness, swelling, and itching most people have where the bite is.
  • Chemicals. You can be allergic to chemicals in cleaners, paints, or soaps. Some people are allergic to latex (such as in latex gloves).
  • Medicines. Some people are allergic to medicines prescribed by a doctor. But you can also be allergic to things you buy at the store. Even herbs can cause problems.
  • Foods. Foods most likely to cause allergies are milk, soy, eggs, wheat, seafood, peanuts, and tree nuts. But you can have an allergy to any food.
Many environmental factors can trigger allergy symptoms. In this WisePatient video, Mount Sinai Medical Center internist Carlos Rios, MD, describes common culprits, from pollen to dust and mold.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.