A food allergy is a reaction of the immune system to a food or food ingredient that most people find harmless. If you eat a food that produces an allergic reaction, your immune system responds by releasing numerous chemicals that cause allergic symptoms. A food allergy is different from a food intolerance, which does not trigger an immune response. A food intolerance (such as lactose intolerance, see page 266) usually arises from an enzyme deficiency and produces symptoms such as stomach cramps, gas, or diarrhea. Food intolerances are relatively common, but a true food allergy is rare, affecting only about 1 percent of the population.
The most common foods that cause allergies in adults are fish and shellfish, eggs, and nuts—such as peanuts, walnuts, and pecans. Symptoms of food allergy can include skin reactions such as hives or rashes, nasal congestion, asthma attacks (see page 245) in people who have asthma, and gastrointestinal problems such as nausea, gas, or diarrhea. Because both food allergies and food intolerances can cause intestinal symptoms, the two disorders are easily confused and must be diagnosed by a doctor.
Food allergy symptoms can appear immediately after eating or may develop over time—within hours or even days. In severe cases the food can provoke a serious reaction known as anaphylaxis (see "Anaphylactic Shock," previous page), which can be life-threatening. Signs and symptoms of anaphylaxis include hives, difficulty breathing, a drop in blood pressure, and loss of consciousness.
To determine whether you have a true food allergy, your doctor probably will perform one of two tests: a skin prick test or a blood test called the radio- 385
allergosorbent test (RAST). During the skin prick test, the doctor deposits a Immune small amount of the suspected food allergen onto your forearm and then pricks system the skin beneath it with a needle. A small red bump will appear at the site if you are allergic to that particular food. For the RAST, a sample of your blood is taken and sent to a laboratory for analysis to determine whether your body has formed antibodies to the food in question. Your doctor also may ask you to record everything you eat in a food diary for a couple of weeks to help in the diagnosis.
Many people will outgrow food allergies, except allergies to nuts, fish, and shellfish. The best treatment for a food allergy is to avoid eating the food that causes the allergic reaction. You will need to read food labels carefully and ask questions when dining out to make sure that the foods you eat do not contain the allergen. Severe allergic reactions are life-threatening. If you have had a previous severe allergic reaction, you will need to carry an injecting device that contains epinephrine with you at all times, so you can inject yourself immediately if you have another allergic reaction.
If you have a food allergy, your immune system mistakenly regards a particular food as harmful and begins producing antibodies to that food. Each time you eat that food, your immune system releases various chemicals to protect your body. These chemicals produce symptoms that can affect your airways, skin, or intestinal tract. Symptoms of food allergy can include:
• skin reactions such as hives or rashes
• nasal congestion
• shortness of breath, or asthma attacks in people who have asthma
If the reaction is severe, you may experience life-threatening symptoms within minutes of eating. A severe allergic reaction can rapidly cause difficulty breathing, lowered blood pressure, and loss of consciousness (see "Anaphylactic Shock," page 383). If you or a person you are with has these symptoms, seek emergency medical help immediately.
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