What Happens

The first time you eat a food that triggers an allergic reaction, your body's immune system recognizes the food as a foreign substance (allergen). Your body reacts by developing antibodies against the food. When you eat the offending food again, the antibodies attack the allergen, releasing histamine and other chemicals that cause the symptoms of an allergic reaction.

Symptoms often develop within minutes, although they may not be apparent for an hour or more. In some cases, symptoms may not develop for days. In general, the sooner the reaction begins, the more severe it will be.

Symptoms may start with tingling and swelling of the mouth and tongue. When the food allergen enters the stomach and intestines, cramping, nausea, vomiting, and diarrhea can develop. Symptoms may occur throughout the body when the food allergen is absorbed from the digestive system into the bloodstream.

In moderate reactions, the body reacts with hives and itching, wheezing, nasal congestion, and possibly dizziness or lightheadedness. Most of the symptoms of a moderate allergic reaction subside in a few hours after antihistamine medicine is taken or the food allergen leaves the body. A skin rash (atopic dermatitis) may persist for several weeks.

In severe reactions, the body releases larger amounts of histamine and other chemicals that can cause life-threatening anaphylaxis. This may happen in a few minutes to an hour. Symptoms of anaphylaxis usually get better quickly after epinephrine and antihistamines are given. Epinephrine, which is given as a shot, stops the further release of the chemicals that cause a severe reaction and counteracts the symptoms.