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Severe Allergic Reaction (Anaphylactic Shock) (cont.)

What Are Causes and Risk Factors for a Severe Allergic Reaction?

An anaphylactic reaction occurs when the body's immune system overreacts to an antigen, which it recognizes as an "invader" or foreign substance.

  • The body's white blood cells produce substances called antibodies as a reaction to that antigen. The antibodies circulate in the bloodstream and attach themselves to certain cells in the body.
    • In an allergic reaction, the antibody is called immunoglobulin E, or IgE.
    • When the antibodies come in contact with the antigen, they signal other cells to produce certain chemicals called "mediators." Histamine is an example of a mediator.
    • The effects of these mediators on organs and tissues of the body cause the symptoms of the reaction.
  • Triggers of anaphylaxis include many substances. Only a trace amount of the trigger may be needed to cause a severe reaction. Triggers of allergic reactions, including anaphylaxis, may include:
    • Prescription and over-the-counter medications (see Drug allergy)
    • Venom of stinging insects such as yellow jackets, bumble bees, honey bees, wasps, fire ants (see Allergy: Stinging Insect Venom)
    • Foods, especially high-protein foods - most commonly, shellfish, fish, nuts, fruit, wheat, milk, eggs, soy products (see Food allergy)
    • Food additives, such as sulfites
    • Transfusion of blood or blood products
    • Numerous other substances such as latex (natural rubber)
    • Dyes and contrast materials used during radiologic procedures or tests
  • Sometimes the trigger of the reaction is obvious--a bee sting, or a new prescription drug. Often, however, the trigger is unknown.
  • People with asthma, eczema, or hay fever are slightly more likely to have an anaphylactic reaction than people who do not have these conditions.
Medically Reviewed by a Doctor on 11/20/2017
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