From 3 percent to 8 percent of children have reactions to some foods. Only 1 percent to 2 percent have true food allergies. Some children seem to grow out of their sensitivity to certain foods, often by age 4. Allergies to peanuts, tree nuts and shellfish usually do not go away, though.

From 1 percent to 2 percent of adults have true food allergies. But people of any age can have sudden allergic reactions to a food that had previously not been a problem for them.

If you have an allergy, a reaction is triggered within minutes to two hours after you consume the allergen. How soon and how severe the reaction is depends on how sensitive you are to the food, the amount of the food consumed, other food consumed, the manner in which it is prepared (i.e., cooked or uncooked, seasoned or unseasoned), and any other medical problems you have.

Severe, life-threatening reactions are more common with allergies to peanuts, tree nuts, shellfish, fish and eggs. These life-threatening reactions are more common in people who also have asthma.

Reactions can affect different body systems:

• The digestive tract, which first comes into contact with food. Some symptoms, such as swelling and itching of the lips, the lining of the mouth as well as throat tightness and hoarseness may occur quickly. When the food enters the stomach and then the intestines, nausea, cramping, pain, vomiting and diarrhea may occur.
• Body systems, such as the skin, lungs and blood vessels, that are affected after the food leaves the digestive tract. These reactions can occur in minutes or within two hours. Often, hives and swelling of the skin occur. Anaphylaxis, the most dangerous and life-threatening result of a food allergy, usually occurs within minutes after consuming the food. When this happens, blood vessels widen so much that blood pressure falls. Symptoms include wheezing, difficulty breathing, throat tightness, nausea, rapid pulse, flushing, faintness, itching of the palms and sole of the feet and even passing out. Without speedy treatment, this intense allergic reaction can cause death. (See the Asthma and Allergy Answers article, "What is Anaphylaxis").

The first severe reaction to a food may be unexpected. Sometimes the patient may at first have minor symptoms such as stomach cramping or hives.

If your doctor suspects you may have a food allergy, the first step is to take a detailed medical history and physical exam. Other tests are used to confirm that you are allergic to certain foods. Some tests use extracts of the suspected foods:

• The skin test involves scratching or pricking your skin with one or more extracts. If your body makes an antibody to the food, redness, itching and minor swelling will occur at the test site. However, the tests are not 100 percent accurate. A positive skin test supports the diagnosis of food allergy, but still does not guarantee that the symptoms are caused by the allergy. In fact, many people regularly eat foods that they have tested positive, but have never experienced a reaction. A negative test does not completely rule out the possibility that allergy is the problem. The accuracy of the skin tests for food allergies varies greatly with the particular food being tested.
• RAST (radioallergosorbent test) is a blood test done in a laboratory. It is used to test a sample of blood for antibody to a specific food. While more costly and less sensitive than skin testing, RAST is particularly useful when eczema and other skin conditions make skin testing difficult. It also may be used to confirm a diagnosis when there is a risk of an anaphylactic reaction to skin tests.
• Another test is the oral food challenge. This test provides the most convincing results. It is required if the relationship between the eating a specific food and symptoms is still unclear after skin tests. Your doctor will explain that all oral challenges -- giving patients the suspected foods -- carry a risk of causing an allergic reaction. They should be done with a specialist physician present and in a setting where allergic reactions can be treated promptly.
• If the diagnosis is still unclear, you may be put on an elimination diet. The first step is to follow the usual diet for 10 to 14 days. You keep a record of what and how much you eat, when a reaction occurred, and what the reaction was. The foods suspected of causing the reaction are then removed from your diet. Make sure that the foods are not in other foods you eat. For example, egg or milk may be in mayonnaise or salad dressings. Elimination diet should only be used for a limited period of time such as 10-14 days. If you have symptoms, or if multiple food sensitivities are suspected, you will be referred to a specialist for further evaluation and treatment.

If you still have symptoms, or if multiple food sensitivities are suspected, the doctor may put you on a strict commercial diet preparation to eliminate most foods. Food may then be put back in your diet. If your symptoms are significant, this is done only with a doctor present.

Several factors make diagnosis difficult. The reaction may depend on the amount of food consumed, the presence of other foods that can slow digestion, and medications such as antihistamines that may hide reactions.The proteins -- the antigens within the food or drink that cause the allergy -- may be altered by cooking or processing in some way. The antigens may be in only part of the food, such as the skin of an apple. Some are present only at a particular stage of ripeness.

Reaction apparently due to a food or food additive may in reality be due to another food that was accidentally added to the mixture during preparation.

Toxins and food poisoning can cause symptoms that can be confused with food allergy.

Some foods upset the stomach and resemble food allergy. Examples are prunes, soybeans and onions. Some medical conditions such as hiatal hernia, ulcers and diverticulosis are associated with acute symptoms after eating.

Some people can't digest lactose, because they don't produce enough lactase, and may have symptoms after drinking milk. The reactions may be confused with food allergy.

To prevent or modify the development of food allergy, identify early in life people who are most at risk:

• Those with a family history of allergy
• Babies with allergy antibodies in their umbilical cord blood or serum
• Infants less than 12 months old with antibodies to egg and other foods including peanut, codfish and milk

Consult a doctor about whether to test an infant for allergy antibodies. If positive, talk with the doctor about how to decrease the incidence and severity of the food allergy.

Allergic reactions to cow's milk or soy formula can appear within days or months after birth. There is evidence that infants who are breast-fed exclusively during their first six to 12 months of life develop fewer allergies by age one or two than infants fed with formula. The American Academy of Pediatrics (AAP) recommends exclusive breast-feeding as ideal nutrition for about the first six months of life. Furthermore, a maternal diet that avoids eggs, cow milk, peanuts and fish while nursing may help reduce eczema in infants.

• Do not consume foods that cause a reaction. People with a severe allergy can go into anaphylactic shock from trace amounts of the food to which they are allergic. Touching foods cause some people to have a severe reaction.
• Read the ingredients lists on food labels to make sure allergy-causing foods are not mixed in. Read the list even if you have had the product before. Ingredients may change.
• If you are traveling, send special foods ahead. Stay in hotels with kitchenettes so you can prepare your own food.
• When eating out, always ask restaurant staff about ingredients in food and how it was prepared. Cooking oils can have allergens. Peanut oil is often used in cooking, particularly in Thai cuisine.
• For infants, elemental formulas or formulas with altered protein should prevent food reactions. Discuss the various formula options with your doctor. Do not assume products labeled "hypoallergenic" will not cause a reaction.

If you have a severe reaction, take medication and seek medical care promptly. Injectable epinephrine, such as EpiPen or Ana-Kit, should always be at hand for treating anaphylactic shock. Get medical care promptly after using epinephrine, even if you feel better. Symptoms may reoccur in a few hours.

Antihistamines and steroids also may be taken to lessen symptoms. Prompt treatment often can limit the severity of the reaction. If you have life-threatening allergies, wear a Medic-Alert bracelet to let health care workers know of your allergy in an emergency.