Food allergies are serious business. According to Dr. Michael C. Young,assistant clinical professor at Harvard Medical School, approximately 11.4 million Americans (4 percent) have food allergies, with 30,000 emergency room visits and 150 to 200 fatalities per year attributed to them. And food allergies are on the rise. A recent study shows a doubling of peanut allergies in children in the past five years, says Young. There is no cure, and the only way to prevent reactions is to avoid the offending food.

Here are answers to common questions about food allergies.

Q. How do you know you have a food allergy?

A. It's tricky to identify an allergenic food after one isolated reaction. The usual culprits include peanuts, nuts, seeds, fish and shellfish. If more reactions occur, there is typically a pattern that makes the food easier to identify, says Dr. Anna Nowak-Wegrzyn, a professor at the Mount Sinai School of Medicine. Diagnosis requires laboratory testing such as skin-prick tests and measuring antibody levels. "Frequently, oral food challenges (ingesting the suspected food under physician supervision) are needed to conclusively establish the diagnosis," adds Nowak-Wegrzyn.

Q. How do you recognize an allergic reaction?

A. "Allergic reactions typically occur within minutes to no more than one to two hours after ingestion and contact with the food," says Young. Ninety percent of cases produce skin reactions, 47 percent induce respiratory responses, 35 percent result in gastrointestinal symptoms and about 20 percent result in anaphylaxis, says Young.

Q. What's the difference between food intolerance and allergies?

A. Allergic reactions are immunologic responses to food proteins that trigger the release of histamines, which result in itching, rashes and hives, swelling, wheezing, vomiting, anaphylaxis, etc. "Food intolerance results from the inability of the gastrointestinal tract to digest and metabolize components of the food, such as lactose intolerance for milk products," says Young.

Q. What creates a food allergy?

A. According to Young, "Some individuals are prone to making immunoglobulin E (IgE) antibodies, which target specific food proteins. When the food proteins are ingested, these IgE antibodies bind to them and trigger the release of chemical mediators, such as histamine, from many tissues of the body, including the skin, lungs, circulatory system, digestive tract, mucous membranes of the nose, sinuses and throat." Histamine causes tissues to swell, itch and secrete mucus, airways to contract, blood vessels to dilate and blood pressure to drop, and, in the worst case, anaphylaxis.

Q. What is anaphylaxis?

A. An allergic reaction that ranges from mild to life-threatening. It is most commonly seen as the effect of the rapid release of histamine, which can cause hives, swelling, throat itching or closure, low blood pressure and dizziness, as well as vomiting or cramping and diarrhea, says Dr. Jonathan Becker, a professor of medicine at the University of Washington. Anaphylaxis occurs in 40 to 50 percent of individuals with food allergies, adds Young.

Q. What are the most common food allergies?

A. Cow's milk, eggs, peanuts, soy, wheat, tree nuts, fish and shellfish account for 90 percent of childhood allergies.

Q. Are some allergies worse than others?

A. Any food may cause fatal anaphylaxis, but peanuts and tree nuts are the most frequent causes of life-threatening anaphylaxis, says Nowak-Wegrzyn.

Q. How can you ensure restaurant food is safe?

A. Contact the restaurant in advance, and make sure it's a food allergy-friendly establishment. Be clear that this is about a life-threatening issue not pickiness. When you arrive, talk to the manager and/or chef. "Inquire about how meals are cooked. Ask them to pay particular attention to not sharing utensils, pots or pans that may be used for the allergenic food, and order only meals whose ingredients you can easily identify. Grilled meats are generally safer than meats in sauces," says Nowak-Wegrzyn.

Cross-contamination is tough to prevent. Mixed use of deep fryers for seafood and nonseafood items, shared utensils and buffets with many different types of foods all create complications, says Young.

Q. Can you develop food allergies later in life?

A. Yes, it is possible, especially to fish, shellfish, peanuts or tree nuts. It is extremely unlikely to develop an allergy to milk, eggs or wheat in adulthood, except in the setting of heavy occupational exposure such as in bakers, who are exposed to these food proteins in the air, says Nowak-Wegrzyn.

Q. Is any one group of people more susceptible to food allergies than others?

A. Children younger than 3 who have signs of being allergic, such as eczema, asthma, nasal and sinus symptoms, and who have relatives with allergic problems are a highly susceptible group, says Young.

Charles Stuart Platkin is a nutrition and public-health advocate, and author of "Breaking the FAT Pattern" (Plume, 2006). Sign up for the free Diet Detective newsletter at www.dietdetective.com.