The accurate diagnosis of food allergy is critically dependent on a good history. Up to 20% of the population may perceive food as a cause of their symptoms, whereas the prevalence of true food allergy is around 1%; food allergy tends to occur in highly atopic subjects with a strong personal and family history of allergies. A clear association between ingestion (or contact) with the food and symptoms may be elicited. Only a limited number of foods commonly provoke symptoms: in children the culprits are eggs, milk, and peanuts; in adults they are fish, shellfish, fruit, peanuts, tree nuts, etc. (Fig. 1.7). Frequently, more than one organ system is involved; i.e. true food allergy is a rare cause of isolated asthma in adults, although severe foodinduced allergy may provoke asthma associated with other typical organ involvement, e.g. lip tingling, angioedema, nettle rash, nausea, and vomiting.
This is in contrast to the typical patient presenting with non- IgE-mediated food intolerance; the symptoms tend to be nonspecific or confined to one organ. There is often no clear history of provoking foods. Alternatively, atypical foods, such as yeast and wheat, are perceived to be involved, with no clear association between ingestion and exposure or delayed symptoms following ingestion. Such patients are either non-atopic or the symptoms occur independently of their atopic status; the latter patients, unlike those with typical food allergy, are unlikely to be highly atopic on the basis of their personal or family history, or via the detection of allergen-specific IgE on skin prick testing or RAST testing.
Non-IgE-mediated food-induced reactions may occur following the ingestion of preservatives such as salicylates, benzoates, and tartrazine. Common products containing preservatives include meat pies, sausages, cooked ham and salami, colored fruit drinks, confectionery, and wine (Fig. 1.8).
No diagnostic tests are available and diagnosis depends upon the history and observation of the effect of exclusion diets and, where necessary, blinded food challenges.
Several clinically relevant cross-reactions may occur between certain inhalant allergens and foods (Table 1.6). A common example is oral allergy syndrome in patients with springtime hayfever (i.e. sensitivity to birch pollen) and oral itching and lip swelling on eating apples (particularly green apples), hazelnuts, and stone fruits (peaches, plums, etc.). Such reactions tend not to be severe and cooked fruits are well tolerated, indicating the labile nature of the allergens responsible.
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