Anaphylaxis is a rare but severe allergic reaction that can affect the skin, lungs, nose, throat, gastrointestinal tract and heart. Roughly 30 out of 100,000 people will experience anaphylaxis. It is a systemic (involving more than one body system) allergic reaction that occurs after a person has been sensitized to a particular allergen, and subsequently encounters that allergen at a later time.

Because anaphylaxis is an allergic reaction, a person usually does not experience an attack the first time they encounter the allergen. An initial encounter creates the opportunity for sensitization, in which the immune system mistakenly recognizes the allergen as a harmful substance and creates immunoglobulin E (IgE) antibodies. These antibodies trigger the allergic reaction the next time the individual encounters the allergen. Frequently, an individual is unaware of the sensitization process as it is taking place and may not remember having been exposed to the allergen prior to an episode of anaphylaxis.

During an allergic reaction, the body recognizes the allergen as an antigen, or foreign invader. IgE antibodies that had been created during sensitization, and had attached themselves to cells in the body called mast cells and basophils, now go into action. When IgE comes into contact with the antigen, it signals the cells to which it is attached to release chemical mediators such as histamine.

These chemical mediators cause inflammation, a protective response of the immune system designed to help drive the antigen out of the body. In addition, the chemicals trigger other responses in the tissues that lead to other symptoms of an allergic reaction.

In a typical allergic reaction, only certain cells in a section of the body react. In anaphylaxis, two or more body systems undergo an allergic reaction. As the tissues of the body release histamine and other substances, the airways constrict. Blood vessels dilate, lowering blood pressure severely and causing lightheadedness. Fluid leaks from the bloodstream into the tissues, resulting in lower blood volume. Bronchial tissues swell, causing breathing difficulties.

These changes lead to anaphylactic shock. As blood pressure plummets, the heart stops pumping blood to the tissues, a condition known as cardiovascular collapse. Tissues and organs fail to get the fluids and oxygen they need to survive. Sometimes fluid leaks into the alveoli (air sacs) of the lungs, causing pulmonary edema (fluid and swelling in the lungs).

An immediate epinephrine injection is necessary to reverse these symptoms. Without it, anaphylactic shock can be fatal. Anyone who suspects they are having an anaphylactic reaction should take epinephrine immediately and call an ambulance for additional medical attention.

There are five primary types of triggers for anaphylaxis:

• Food allergies
• Drug allergies
• Latex allergies
• Insect stings
• Idiopathic triggers

About 50 percent of anaphylactic attacks are idiopathic, in which a trigger cannot be identified. Sometimes, a trigger will become evident after subsequent anaphylactic episodes.

Although anaphylaxis is not a common condition, those with asthma as well as allergies may be at increased risk for having an anaphylactic reaction. In addition, patients who have had prior reactions are at increased risk for subsequent anaphylactic reactions.