Allergies occur when an overactive immune system produces proteins called IgE antibodies against substances in the environment that are otherwise harmless — pollens, house dust mites, moulds, animal hair (dander), and in some cases, specific food proteins. Food allergy is mainly a problem of infants, toddlers and young children. Over 90% of cases are associated with atopic eczema — an intensely itchy chronic skin rash affecting the face, arms, legs, and other parts of the body — and there is usually a family history of allergic disorders such as asthma, hay fever and eczema.
In adults, a single food such as peanut or shellfish is usually involved, but children are commonly allergic to two or three foods, sometimes more. The most common ones are peanut, egg, milk, other nuts, seafoods and/or sesame. Fortunately, most children grow out of their egg and milk allergies before they reach school age, or during the early school years, but allergies to nuts and seafoods can persist. Wheat and soy can cause allergies, but they tend to be mild and transient.
New food allergies can occasionally arise in adult life, usually with crustaceans and other more exotic foods that are not eaten in childhood. With rare exceptions, reactions to fruits, vegetables, herbs and spices are due to chemical intolerances rather than allergies. Skin prick tests or blood tests (radioallergosorbent test or RAST) can detect IgE antibodies in people with allergies, but are of no value for diagnosis of intolerances.
Food allergy reactions
Food allergy reactions vary in severity, depending on how sensitive the person is and how much of the food they’ve eaten. Mild reactions may only cause a little eczema or a few hives, and the cause may not be obvious. More severe reactions are usually obvious and occur consistently, every time the person has the food. Contact with the mouth and tongue can cause an immediate burning sensation, with hives and redness around the face and, if the food is swallowed, an immediate feeling of being unwell can be followed by vomiting, cramps and diarrhoea. The face, mouth and eyes can swell dramatically, and hives on the body can join into large, rapidly spreading welts.
The most severe type of reaction — anaphylaxis — can progress rapidly with breathing difficulty (from swelling of the throat or severe asthma), allergic shock and collapse, and can be life-threatening if not treated immediately with adrenaline (epinephrine) by injection. In the most sensitive people with a food allergy, tiny amounts of the food (pin-head sized) can be enough to provoke a severe reaction.