All about allergy

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.

Allergy journal

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