The most serious allergic reaction to food is anaphylaxis (also known as anaphylactic shock). Like other, less serious reactions, it does not typically occur upon the first exposure to the allergen, but only after the individual has been sensitized. Anaphylactic shock is most commonly caused by peanuts, tree nuts, shellfish, milk, eggs, and fish, though any food can cause it. Quantities of the allergen measuring significantly less than a teaspoon have caused death. Symptoms, which are quite severe, can occur within five minutes of eating, though reactions that are life threatening may take place over the course of several hours. Symptoms include:

• Difficulty breathing
• Severe panic
• Swelling of mouth and throat
• A drop in blood pressure
• Loss of consciousness

The sooner an anaphylactic reaction is treated, the great the likelihood of survival. Individuals experiencing anaphylactic shock should be immediately taken to the emergency room, even if they appear to be recovering.