For an anaphylactic reaction to occur, you must have been exposed in the past to the substance that causes the reaction, called the antigen. This is called sensitization. A bee sting, for example, may not cause an allergic reaction the first time. Another bee sting may produce a sudden, severe allergic reaction known as anaphylaxis or anaphylactic shock. These reactions usually occur within seconds to minutes of exposure. Occasionally, they are delayed.

You may develop sensitivity and anaphylaxis to a substance that you have been exposed to many times in the past without a reaction, and often people don't recall the previous exposure. An anaphylactic reaction occurs when the body's immune system overreacts to an antigen, which it recognizes as an invader or foreign substance. A venom is a poison, particularly one secreted by an animal. For examples, bee venom, snake venom, scorpion venom, and spider venom. Snake venom is also called venin.

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, venom of stinging insects such as yellow jackets, bumble bees, honey bees, wasps, fire ants.

Foods, especially high-protein foods - most commonly, shellfish, fish, nuts, fruit, wheat, milk, eggs, soy products and 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.