A food allergy is a reaction by the body's immune system to something in a food, usually a protein. The body mistakes the food for a germ or some other foreign invader and tries to defend itself.
While the exact cause of food allergy is unknown, it does tend to run in families.
Any food can lead to an allergic reaction, but some foods are much more likely to do so.
The following food are the most common triggers of food allergies in children:
- Milk, and dairy products containing milk, such as cheese and yogurt.
- Wheat. Note: wheat allergy is different from celiac disease.
- Shellfish, such as crab, lobster, and scallops
- Tree nuts, such as walnuts, cashews, and brazil nuts
Most allergic reactions occur within 30 minutes of consuming the food. A reaction can occur within 5 to 10 minutes or take as long as 4 to 6 hours to appear after ingestion.
Food allergy is different from food intolerance. In food intolerance, there is a physical reaction to a certain food that is not due to an allergy. Lactose intolerance is a common example of this. People with lactose intolerance cannot digest the sugar in milk (lactose) and have indigestion and feel bloated after drinking milk or eating cheese. This reaction is due to their body's inability to process the sugar, and cannot be treated as an allergy.
Another type of food allergy is oral allergic syndrome, which can cause itching or inflammation around the lips, mouth, and throat after eating certain fruits or vegetables. This is rarely dangerous.
What are the symptoms of a food allergy?
It's not always easy to know when someone has a food allergy, because there are many different symptoms. They include:
- Hives (a raised, pink, itchy rash)
- Scratchy throat
- Stomach ache
- Nausea or vomiting
- Swelling, especially around the mouth and face
- Itchy eyes
- Nasal congestion/runny nose
- Other rashes, including eczema
- Wheezing or trouble breathing
- Difficulty with swallowing
- Rapid heart rate
- In babies and small children, poor growth or blood in the stool
In severe cases, something called anaphylaxis can happen. This whole-body allergic reaction combines many of the symptoms mentioned above and can lead to death if not treated immediately.
How is a food allergy diagnosed?
Sometimes doctors can diagnose food allergy based on the patient's history, such as if a child has broken out in hives after eating peanuts. But symptoms are often varied enough to make diagnosis tricky. While a detailed food diary and family history can be helpful, sometimes doctors need to do tests to be sure. The two most commonly used tests are skin tests and RAST.
In this test, the person's skin is pricked with a solution that contains trace amounts of the suspected food. If there is an allergy present, the skin will show a small hive-like outbreak. The advantage of a skin test is that it's inexpensive, uncomplicated, adn somewhat reliable. However, some children find it uncomfortable and may suffer significant reactions even with such a small amount of food exposure. Also, for the test to be truly reliable, the patient must avoid antihistamines for two weeks, which may not be safe for children with severe allergies.
RAST blood tests
IgE antibodies detect allergens in the body. Radioallergosorbent (RAST) laboratory tests measure the amount of food-specific IgE in the blood. Since these antibodies stay in the body after exposure to an allergen, RAST tests will detect them at any time. The greater the amount of IgE, the higher the probability the person has an allergy to that particular food.
RAST tests use only one needle stick to draw blood and so are less uncomfortable than skin-prick tests, which require many sticks. However, RAST tests are expensive and can product false results.
Elimination and food challenge
Another way to diagnose food allergies is with a test called a double-blind, placebo-controlled (DBPC) food challenge test. In this test, capsules containing the suspected food and others containing sugar are given to a person, and the reaction is observed. Because there can be a risk of a serious, life-threatening reaction, this test usually is done in a clinic or hospital.
A more common way to do this test is for the person to record what he or she eats and monitor any reactions, which typically occur within two hours of ingestion. First, the suspected foods are eliminated from the diet for one to two weeks. Then, the foods are added back into the diet slowly under medical supervision. It's helpful to keep a food diary of everything consumed and the amounts. This approach would only be taken if the reaction was mild and didn't cause any trouble with breathing.
How can the risk of allergic reactions be reduced?
The best way to prevent an allergic reaction is to avoid the food or foods that cause a reaction. Here are some steps you can take to help your child with food allergies:
- Educate yourself so that you can recognize other forms or minor traces of the allergens in the foods your child eats. Read food labels and ask questions when you eat in restaurants. If your child has a milk allergy, for example, avoid foods with ingredients that include casein, caseinate, whey or milk solids. And some foods, while they don't contain peanuts, are manufactured on machines that also manufacture foods with peanuts, making them dangerous to allergic people.
- Educate your child about foods that must be avoided and why.
- Inform all adults who have contact with your child about the allergy and what to do in an emergency.
- Learn cardiopulmonary resuscitation (CPR).
- If your child has a history of severe reactions to certain foods, you and your child should carry epinephrine at all times and use it at the first sign of an allergic reaction. Epinephrine is usually given with an self-administered injections (Epipen*reg;, Twinject®). Ask your doctor for several of them (so that you can leave one everywhere your child goes often, to be safe), and make sure that everyone who cares for your child knows how to use it.
- Make sure your child's school or daycare is aware of the allergy. Even small amounts of peanut butter in a classroom or on a lunch table can be dangerous, for example. Many schools have peanut-free classrooms and lunch tables.
Are food allergies permanent?
While most children outgrow food allergies, some carry them into adulthood. Teenagers need to be aware of food allergies they had as children and not assume they have outgrown them. In particular, allergies to peanuts, tree nuts, fish and shellfish usually are not outgrown.
How are allergic reactions treated?
For minor allergic reactions, over-the-counter or prescription antihistamines or nasal sprays can be used to relieve the symptoms. Talk to your doctor about the best medication for your child.
Severe reactions require prompt medical attention. Epinephrine (usually via the Epipen® or Twinject® mentioned above) can be lifesaving. If the Epipen® is used, the child still needs medical attention, as symptoms can return once the medication wears off. All those caring for the child should be instructed to either call 911 or take the child directly to an emergency room (never take the child yourself if there is any trouble breathing or the child appears ill--always call 911 in that case).
When to call a professional
If you or your child develop allergic symptoms after eating, call a health care professional immediately. If breathing difficulties occur, call 911.