Dear Doctor Column, December 5, 2005

Question:

An hour after eating a crab dinner my eyes began to itch. In the morning, my face and ears were swollen and red and an itchy rash had spread down my neck. I've never had a reaction to crab before. Could I suddenly become allergic? Besides giving up shellfish, is there anything I can do to prevent this from happening again?

Answer:

Any food can cause an allergic reaction, but shellfish are among the most frequent offenders. Other common culprits are eggs, peanuts, milk and soy products, tree nuts, wheat, and fish. Food allergies often begin in childhood, but can appear at any time.

People often assume an unpleasant reaction to food is caused by an allergy, but many actually have food intolerance, which is a much more widespread condition. Food allergies involve the immune system, while food intolerance is caused by problems with digestion or metabolism. People with food intolerance often lack adequate amounts of certain digestive enzymes that normally help metabolize food. Common food intolerances are lactose intolerance – an inability to completely digest milk and dairy products – and wheat intolerance. People with these conditions can experience gas, bloating, and stomach cramps after eating a food they cannot completely digest.

Food allergies, which affect about 2% of adults and up to 8% of children, happen when your body's immune system mistakes a generally harmless food for a dangerous substance, such as a virus or bacteria, and creates antibodies specifically designed to fight that "invader." The next time the food is eaten, the body's immune system recognizes it and mounts a defense, releasing massive amounts of chemicals and histamines. Immune reactions can range from mild to life threatening.

Allergic reactions to foods usually occur soon after eating – generally within 2 hours, though symptoms can appear later. Reactions can involve the respiratory or cardiovascular systems, gastrointestinal tract, or skin. Symptoms can include a runny nose or sneezing, swelling (especially of the lips, tongue, and throat), itching, wheezing or breathing problems, hives or flushing, vomiting, stomach cramps, diarrhea, and fainting.

People with food allergies also are at risk for anaphylaxis, a severe whole-body reaction that usually begins within minutes of eating the offending food. Signs of anaphylaxis include difficulty breathing, wheezing, hoarseness, a tight feeling in the throat or chest, a drop in blood pressure, and loss of consciousness.

Anaphylactic reactions, also known as anaphylactic shock, progress rapidly and can be fatal if not treated right away. People with any symptoms of anaphylaxis need immediate medical treatment. An injection of epinephrine can be a lifesaver by controlling severe reactions. Epinephrine, a synthetic version of the natural hormone adrenalin, quickly constricts blood vessels, raising blood pressure, relieving wheezing, and making breathing easier while stimulating the heart.

People with food allergies should always carry injectable epinephrine, which is available by prescription. An easy-to-use device called an Epi-Pen® (a version called the Epi-Pen® Jr. is available for children) allows people to inject a single premeasured dose of the drug at the first sign of an allergic response. After injecting epinephrine, the person suffering the reaction should be taken, preferably by ambulance, to the nearest emergency department for evaluation and observation, even if symptoms seem to subside.

Unfortunately, the only way to avoid allergic reactions to food is to completely avoid the food you are allergic too. First, you need to positively identify the food allergen. In your case, it sounds like crab may be the culprit, but the only way to know for certain is to consult with a physician, often an allergist or immunologist, who specializes in food allergies.

Your doctor can supervise an elimination diet, where the suspected food is removed from the diet until symptoms disappear, then reintroduced to see if a reaction develops, or perform skin or blood tests. During a skin test, your doctor will prick your back or forearm with a needle, delivering a tiny amount of the suspect substance. If you are allergic, a small bump will form at the injection site.

Blood tests, called RAST or radioallergosorbent tests, require laboratory analysis of a blood sample. At the lab, blood is tested with specific foods to determine whether antibodies to that food are present. Results of the test are usually available in about a week. Keeping a food diary, a written record of everything you eat and drink and any symptoms that develop, may help your physician pinpoint the cause of your allergy. You should never try to cause a reaction or reintroduce a food without medical guidance.

Avoiding foods you are allergic to may seem simple, but it is important to know offending foods can "hide" in other foods as ingredients, and even trace amounts of an allergen are enough to provoke a severe reaction. A person who is allergic to tree nuts, for example, could have a reaction if the cook chops walnuts and then uses the same knife to prepare another part of the meal. It's important to read all labels thoroughly and tell restaurant servers about your allergy so they can check with the chef to make sure your meal is free of any food you are allergic too.

For more information on food allergies, visit the Food Allergy & Anaphylaxis Network and the American Academy of Allergy, Asthma, and Immunology.