Allergy Capital

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Food Allergy - Overview

Severe hives / urticaria (LEFT) and skin testing (RIGHT) in children with food allergy

The term allergy is often misused to describe annoying but harmless symptoms such as headaches or bloating. Food allergy occurs in 5 % young children and 1 % adults. Severe food allergy can trigger life threatening anaphylaxis.

What is food allergy?
The term "allergy" is often misused to describe annoying (but ultimately harmless) symptoms such as headaches after overindulging in chocolate or red wine, or bloating after a milkshake. The result is a widespread impression (in both the lay and medical community) that all food allergies are trivial. Fortunately, the majority of food allergies are not severe, and will disappear with time, particularly in children. When severe reactions do occur, they are frightening for patients and those involved in their care.

What is allergy?
Underneath the lining of the skin, gut, lungs, nose and eyes are mast cells. These are designed to kill worms and parasites. Mast cells are like "land-mines", and contain "bags" filled with irritant chemicals including histamine. Mast cells are armed with proteins called IgE antibodies, which act as remote sensors in the local environment. A person allergic to peanut, for example, will have IgE antibodies capable of Recognizing the shape of peanut protein (the allergen), in much the same way that a lock "recognizes" the shape of a key. When this happens, mast cells are triggered to dump their contents (such as histamine) into the tissues, causing an allergic reaction.

How common is food allergy?
Food allergy occurs in around 1 in 20 children and in about 1 in 100 adults. While most childhood food allergy resolves with age, reactions to nuts, seeds and seafood tend to be life-long.

Symptoms of food allergy
Many allergic reactions are mild and limited to localized hives or swelling. The most dangerous symptoms are breathing difficulties or a drop in blood pressure (shock), either of which can be life threatening. Anaphylaxis is the most severe form of allergic reaction. Other symptoms include swelling of the face or throat, dizziness, difficulty thinking, an intense sense of fear, tightness in the chest, vomiting or diarrhoea.

Common triggers of allergic reactions
Food is the most common cause of severe allergy in young children, particularly cows milk allergy, soy, eggs, nuts and wheat. Nuts, seeds, fish and crustaceans are more common allergens in older children and adults, although other triggers such as spices and herbal medicines have been described.

Identifying the cause
Your doctor will normally ask a series of questions that may help to narrow down the list of likely causes such as foods or medicines consumed that day, or exposure to stinging insects. This approach will also help to exclude conditions that can sometimes be confused with anaphylaxis. Skin prick testing or blood (RAST) allergy testing help confirm or exclude potential triggers.

When does food allergy develop?
One first has to be exposed to something to become allergic to it. It is currently believed that the majority of patients become sensitized after birth to small amounts of food passing intact through breast milk. When a child reacts to their "first" taste of peanut butter or egg, it may, for example, be their 10th or 20th time.

Can food allergies be prevented?
There are few studies of allergy prevention, and even fewer examining food allergies. Advice in this area (while sensible) should therefore be considered unproven at this time

• Breast feed for at least 4 months (preferably at least 6 months)
• Avoid exposure to tobacco smoke and don't smoke during pregnancy
• Delay the introduction of solids until age 4-6 months
• Introduce new foods one at a time so that allergic reactions can be more readily identified
• Introduce rice, vegetables, meat and fruits first
• Delay the introduction of egg until at least 12 months
• Delay the introduction of nuts and seafood until at least 24 months
• There is little or no evidence at this time that severely restricting a mother's diet during pregnancy will prevent food allergy. Indeed, severe restrictions may affect the nutrition of the developing baby.
• There is some evidence that certain dietary restrictions during breast feeding may be of some benefit. Any food avoidance should be done with medical or dietetic advice so that maternal nutrition is not compromised.

The natural history of food allergy
Around 80 % of children allergic to cows milk and soy will be able to tolerate it by the age of 3 years. Egg allergy is generally longer lasting, with about 80 % having grown out of it by age 8 years. Unfortunately, the vast majority of those allergic to nuts or seafood rarely seem to grow out of their problem, leading to the need for life-long avoidance strategies. When food allergy develops for the first time in adults, it usually persists.

What determines the severity of a reaction?

• The severity of the allergy
• The amount eaten
• The form of the food (liquid is absorbed faster)
• Whether it is eaten on its own or mixed in with other foods
• Exercise around the same time as the meal may worsen severity.
• Cooked food is sometimes better tolerated

References

• Sampson HA. Food Allergy. J Allergy Clin Immunol 1999; 103: 717-28 and 981-9.
• Sampson HA et al. Fatal and near fatal anaphylactic reactions to food in children and adolescents. New England Journal of Medicine 1992; 327: 380-384.
• Vickers DW et al. Management of children with potential anaphylactic reactions in the community: a training package and proposal for good practice. Clin Exp Allergy 1997; 27: 898-903.
• Committee Report from the Adverse Reactions to Food Committee of the American Academy of Allergy, Asthma and Immunology. Journal of Allergy and Clinical Immunology 1991; 87: 749-751.
• Bernhisel-Broadbent J. Allergenic cross-reactivity of foods and characterisation of food allergens and extracts. Ann Allergy Asthma Immunol 1995; 75: 295-303.
• David TJ (ed). Food Allergy. J Royal Soc Med 1997; 90: 1-48.