An allergy is a physical reaction to some specific food or substance. If you have an allergy to cats, they'll make you itchy and swollen no matter how much you love to pet them.

If you have an allergy, it can range from mild — itchy eyes in the springtime — to serious — being unable to breathe after eating a peanut. People have allergies to all kinds of things, including milk, chocolate, corn, dust, mold, and just about any kind of animal you can think of. Allergy was originally Allergie, a German word invented by a doctor who combined the Greek roots allos, "other, different, or strange," and ergon, "activity."

Definitions of allergy
  1. noun
    hypersensitivity reaction to a particular allergen; symptoms can vary greatly in intensity
    synonyms: allergic reaction
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    food allergy
    allergic reaction to a substance ingested in food
    atopic allergy, atopy, immediate allergy, type I allergic reaction
    an allergic reaction that becomes apparent in a sensitized person only minutes after contact
    serum disease, serum sickness
    a delayed allergic reaction to the injection of an antiserum caused by an antibody reaction to an antigen in the donor serum
    delayed allergy, type IV allergic reaction
    an allergic reaction that becomes apparent only hours after contact
    type of:
    hypersensitivity reaction
    an inappropriate and excessive reaction to an allergen (as pollen or dust or animal hair or certain foods); severity ranges from mild allergy to severe systemic reactions leading to anaphylactic shock
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