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    Allergic Reaction Causes

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    Allergies: If you have them, you know what happens when they’re triggered. But do you know what sets them off?

    Your immune system protects you from bacteria and viruses. Sometimes it mounts an inappropriate response to a harmless substance. When that happens, your body creates chemicals, like histamine, that trigger an allergic reaction. It's unclear what causes your immune system to make the mistake about certain things.

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    Common allergens include dust mites, pet dander, pollen, peanuts, and seafood. Medications, latex, metals such as nickel, chemicals in household cleaning products, and perfumes and dyes found in beauty products can also set off a reaction in some people.

    Who Will React?

    Allergies affect about 1 in 5 people. Experts don't know why some people react to certain things and others don’t. But having a family history of allergies makes it more likely you’ll get them.

    You can develop an allergy at any age. Something that’s never bothered you before may suddenly cause a reaction.

    Signs of Trouble

    The way your body reacts to an allergen can vary. It could be mild or severe, and might affect different parts of the body.

    Hay fever and other seasonal allergies usually cause mild reactions. Sneezing, watery eyes, coughing, and a runny nose are common. The dander in pet fur can cause the same symptoms. Hives and rashes -- red, itchy spots and patches -- can show up on your skin.

    Allergies can also affect your digestive system. Some foods and medications can cause stomach pain, nausea, vomiting, and diarrhea.

    The most serious type of allergic reaction, called anaphylaxis, can be fatal if you don't get help quickly. It causes blood pressure to drop, airways to narrow, and shock to set in. Signs include trouble breathing, a weak but rapid pulse, and loss of consciousness.

    Common triggers of this severe allergic reaction are insect stings and foods like nuts. A medication such as epinephrine is usually needed to halt this dangerous response. If you have severe allergies, you should carry an epinephrine auto-injector in case you’re exposed.

    What to Do

    If you think you may have allergies, see a board certified allergist. There are blood tests that can tell you what you’re allergic to. Knowing that can help you minimize your exposure to the allergen.

    Depending on the type of allergy, your doctor will recommend treatment that may include an oral antihistamine, regular allergy shots, or carrying an epinephrine auto-injector with you.

    WebMD Medical Reference

    Reviewed by Luqman Seidu, MD on April 24, 2016
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