When a person has a food allergy, the body’s immune system mistakenly treats certain substances in that food—called allergens—as harmful. An allergic reaction happens when the immune system tries to fight off the allergen.

The first time a person eats a food he or she is allergic to, there is no allergic reaction. But, the immune system reacts to the food as if it was harmful and makes antibodies to fight that allergen. The next time the person eats that food, an allergic reaction occurs. It may happen within a few minutes to several hours after exposure to the allergen. Sometimes a person does not eat the allergenic food, but is exposed to it by touching it, using a skin or hair care product containing it, or being close to someone eating it.

During an allergic reaction to a food, the following symptoms may occur:

  • Itching in the mouth
  • Swelling of lips and tongue
  • Vomiting, diarrhea, or abdominal cramps and pain
  • Hives
  • Worsening of eczema
  • Tightening of the throat or trouble breathing
  • Drop in blood pressure

The most severe allergic reaction is called anaphylaxis. This is a potentially life-threatening reaction that requires immediate medical attention because it can lead to death if breathing and blood circulation become severely restricted. Anaphylaxis has many symptoms that are similar to less serious allergic reactions, but a combination of the most severe symptoms can indicate it.

Symptoms of anaphylaxis include:

  • Skin symptoms listed above or swollen lips
  • Difficulty breathing
  • A drop in blood pressure
  • Vomiting, diarrhea or cramping

Food allergies are more common in young children than in adults. In the United States, almost 1 in 20 children under age 5—and almost 1 in 25 adults—are allergic to at least one food. People tend to outgrow some food allergies, such as milk, egg and soy. Others, like peanut allergy, can last for life.

Some people can develop a food allergy later in life. For example, milk allergy tends to develop in children, whereas shrimp allergy generally develops in adulthood. Studies show that food allergies are becoming more common, especially peanut allergy.