Severe allergic reaction (anaphylaxis)

Anaphylaxis is a term used to describe a severe allergic reaction. It is not common, but it can be life-threatening and requires emergency care. Symptoms may appear in a few seconds or minutes or be delayed for several hours after contact with the substance that caused the reaction. The sooner symptoms appear, the more severe the reaction is likely to be.

Symptoms or conditions that may occur when a person has a severe allergic reaction include:

• Shock.
• Swelling of the lips, tongue, mouth, throat, neck, ears, eyelids, palms, or soles of the feet (angioedema). Swelling is most serious when it involves the airway and interferes with breathing.
• Coughing, wheezing, difficulty breathing, or a feeling of fullness in the mouth, throat, or neck.
• Lightheadedness, confusion, or agitation.
• Nausea, diarrhea, and stomach cramps.
• Hives and flushing of the skin. These symptoms often occur with other symptoms of a severe reaction.

The severity of the symptoms depends on several factors, including:

• Previous reactions to the same substance (allergen).
• The amount of the substance the person was exposed to.
• The length of time the person was exposed to the substance.

Common causes of severe reactions include exposure to:

• Foods such as peanuts, shellfish, nuts, wheat, and eggs.
• Drugs such as penicillin, cephalosporins, insulin, sulfa drugs, and aspirin.
• Blood products.
• Vaccines.
• Enzymes such as trypsin and streptokinase and enzymes in intravenous (IV) contrast dyes, especially dyes containing iodine.
• Insect bites or stings, including those of yellow jackets, honeybees, hornets, wasps, ants, ticks, and fire ants.
• Jellyfish and Portuguese man-of-war stings.