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Many people believe they're allergic to specific foods. But true food allergies aren't nearly as common as you might think. About 1 percent of adults and 5 percent of children actually have a true food allergy — an adverse reaction to a food that is triggered by the immune system. Far more people have a food intolerance, an unpleasant reaction to food that doesn't involve the immune system.

This is little consolation if you've had a bad experience with a particular food and fear a recurrence. But it is an important distinction, because a true food allergy — also called a hypersensitivity — can cause serious problems and even death.

If you experience a reaction to a particular food, tell your doctor about it. Tests can help diagnose a food allergy, so you can take steps to avoid a repeat problem.

For most people, an adverse reaction to a particular food is uncomfortable. But, for some people, a food reaction can be frightening and even life-threatening. Symptoms of a food allergy usually develop within about an hour after eating the offending food.

The most common signs and symptoms of a true food allergy include:

• Hives, itching or eczema
• Swelling of the lips, face, tongue and throat, or other parts of the body
• Wheezing, nasal congestion or trouble breathing
• Abdominal pain, diarrhea, nausea or vomiting
• Dizziness, lightheadedness or fainting

In a severe allergic reaction to food — called anaphylaxis — you may have more extreme versions of the above reactions, or you may experience the following life-threatening signs and symptoms:

• Constriction of airways, including a swollen throat or a lump in your throat, that makes it difficult to breathe
• Shock, with a severe drop in blood pressure
• Rapid pulse
• Dizziness, lightheadedness or loss of consciousness

As many as 150 to 200 people die of food-related reactions each year in the United States.

Oral allergy syndrome
A mild type of food allergy known as oral allergy syndrome causes tingling or itching around the mouth after eating certain fresh fruits and vegetables. It's caused by cross-reactivity between proteins in fresh fruits and vegetables and those in certain pollens. For example, if you're allergic to ragweed, you may also react to melon.

In a true food allergy, your immune system mistakenly identifies a specific food or component of food as a harmful substance. Your immune system triggers certain cells to produce immunoglobulin E (IgE) antibodies to fight the culprit food or food component (the allergen). The next time you eat even the smallest amount of that food, the IgE antibodies sense it and signal your immune system to release histamine and other chemicals into your bloodstream.

These chemicals cause a range of allergic signs and symptoms. Histamine is partly responsible for most allergic responses, including dripping nose, itchy eyes, dry throat, rashes and hives, nausea, diarrhea, labored breathing and even anaphylactic shock.

The great majority of food allergies are triggered by certain proteins in:

• Eggs
• Peanuts
• Fish
• Shellfish, such as shrimp, lobster and crab
• Tree nuts, such as walnuts and pecans

In children, food allergies are also commonly triggered by proteins in these foods:

• Cow's milk
• Wheat
• Soybeans

Chocolate, long thought by some parents to be a culprit among children, seldom is a cause of allergy.

Food intolerances and other conditions: Not food allergies
Other reactions to food don't involve your immune system or, consequently, the release of histamine. These reactions aren't true food allergies. Instead, they may be food intolerances. Because food intolerances may involve many of the same signs and symptoms as food allergies do — such as nausea, vomiting, cramping and diarrhea — people often confuse the two.

If you have a food intolerance, you usually can eat small amounts of problem foods without a reaction. By contrast, if you have a true food allergy, even a tiny amount of food may trigger an allergic reaction.

One of the tricky aspects of diagnosing food intolerance is that some people are sensitive not to the food itself but to a substance or ingredient used in the preparation of the food. For example, sulfite preservatives can trigger asthma in sensitive people.

Common causes of food-related problems include:

• Absence of an enzyme needed to fully digest a food. You may not have adequate amounts of some enzymes needed to digest certain foods. Insufficient quantities of the enzyme lactase, for example, make it difficult to digest lactose, the main sugar in milk products. Lactose intolerance can cause bloating, cramping, diarrhea and excess gas.
• Irritable bowel syndrome. Certain foods may trigger the signs and symptoms of irritable bowel syndrome. You may find that certain foods will cause cramping, constipation or diarrhea. You may avoid these foods to avoid the symptoms.
• Food contaminated by a toxin. Sometimes food poisoning can mimic an allergic reaction. Some types of mushrooms and rhubarb, for example, can be toxic. Bacteria in spoiled tuna and other fish also can make a toxin that triggers adverse reactions.
• Recurring stress or psychological factors. Sometimes the mere thought that a food may make you sick actually does. The reason is not fully understood.

Certain factors may put you at greater risk of developing a food allergy:

• Family history. You're at increased risk of food allergies if other allergies, such as hay fever, asthma, hives or eczema, are common in your family.
• Age. Food allergies are most common in children, especially toddlers and infants. As you grow older, your digestive system matures, and your body is less likely to absorb food or food components that trigger allergies. Fortunately, children typically outgrow allergies to milk, soy, wheat and eggs. Severe allergies and allergies to nuts and shellfish are more likely to be lifelong.

See a doctor or allergist if you experience food allergy symptoms shortly after eating. If possible, see your doctor when the allergic reaction is occurring. This will aid in making a diagnosis.

Seek emergency treatment if you develop any signs or symptoms of anaphylaxis.

The following may help determine if you have a true food allergy, rather than a food intolerance, food poisoning or some other gastrointestinal condition.

• Description of your symptoms. Be prepared to tell your doctor a history of your symptoms — which foods, and how much, seem to cause problems — and whether you have a family history of food allergies.
• Food diary. Your doctor may ask you to keep a food diary of your eating habits, symptoms and medications to pinpoint the problem.
• Elimination diet. For a week or two, you may be asked to eliminate suspect foods and then add the items back into your diet one at a time. This process can help link symptoms to specific foods. However, this isn't a foolproof method. Psychological factors as well as physical factors can come into play. For example, if you think you're sensitive to a food, a response could be triggered that may not be a true allergic one. If you've had a severe reaction to foods, this method cannot be used.
• Physical examination. A careful exam can identify or exclude other medical problems.
• Skin test. A skin prick test can determine your reaction to particular foods. In this test, small amounts of suspected foods are placed on the skin of your forearm or back. Your skin is then pricked with a needle, to allow a tiny amount of the substance beneath your skin surface. If you're allergic to a particular substance being tested, you develop a raised bump or reaction.
• Blood test. A blood test can measure your immune system's response to particular foods by assessing the amount of allergy-type antibodies in your bloodstream, known as immunoglobulin E (IgE) antibodies. For this test, you give a blood sample in your doctor's office. Your sample is then sent to a medical laboratory, where different foods can be tested. However, blood tests aren't 100 percent accurate.

The only way to avoid an allergic reaction is to avoid foods that cause signs and symptoms.

However, despite your best efforts, you may come into contact with a food that causes a reaction. In this case, you may need an emergency injection of adrenaline (epinephrine) and a trip to the emergency room. In fact, if you have a severe allergy, you may need to carry injectable epinephrine with you at all times.

For less severe allergies, your doctor may prescribe antihistamines, which can be taken after exposure to an allergen to control your reaction and help relieve discomfort. Creams may relieve skin reactions.

The best way to prevent an allergic reaction is to know and avoid foods that cause signs and symptoms. For some people, this is a mere inconvenience, but others find it a greater hardship. For instance, if nuts are a staple part of your diet, it can be a challenge to come up with a substitute that has equal nutritional value. Also some foods — when used as ingredients in certain dishes — may be well hidden. This is especially true in restaurants and in other social settings.

If you know you have a food allergy, follow these steps:

• Know what you're eating and drinking. Be sure to read food labels carefully.
• If you have already had a severe reaction, wear a medical alert bracelet or necklace.
• Talk with your doctor about carrying emergency medications.
• If you have asthma, be sure to ask if sulfites have been added to foods when dining in restaurants. When shopping for foods, check labels for the terms sodium bisulfite, potassium bisulfite, sodium sulfite, sulfur dioxide and potassium metabisulfite.

If your child has a food allergy, take these precautions to ensure his or her safety:

• Notify key people that your child has a food allergy. Talk with child care providers, school personnel, parents of your child's friends, and other adults who regularly interact with your child. Emphasize that an allergic reaction can be life-threatening and requires immediate action. Make sure that your child also knows to ask for help right away if he or she reacts to food.
• Explain food allergy symptoms. Teach the adults who spend time with your child how to recognize signs and symptoms of an allergic reaction.
• Write an action plan. Your plan should describe how to care for your child when he or she has an allergic reaction to food. Provide a copy of the plan to your child's school nurse and others who care for and supervise your child.
• Have your child wear a medical alert bracelet or necklace. This alert lists your child's allergy symptoms and explains how others can provide first aid in an emergency.

Preventing children from developing food allergies
If your baby or child develops a food allergy, it's not your fault. However, these steps may help prevent food allergies:

• Breast-feed your baby. If possible, breast-feed your baby exclusively for four to six months. Your breast milk can help prevent food allergies.
• Don't introduce solid foods too early. Try to wait until six months before giving your baby solid foods.
• Wait on foods that are common causes of allergies. Put off cow's milk until baby's first birthday. Introduce eggs at age 2. And don't offer peanuts, nuts or fish until age 3.
• Watch your diet during pregnancy. Consider cutting out peanuts while you're pregnant, especially if you have a family history of food allergies.