Urticaria may be classified on the basis of underlying aetiology or by a clinical classification. In most cases of chronic urticaria it will not be possible to identify the underlying cause, i.e. the rash is idiopathic; however, when seeing a patient, it is helpful to use a clinical classification, as this guides history-taking and ensures that in any one individual all possible triggers for the urticaria are identified. In some patients several factors may cause/worsen the rash.Acute urticaria caused by SIgE against food protein antigens is often identified easily by the patient, particularly as there may be oral symptoms as the food is eaten and as repeated exposure to the foodstuff may lead to progressively more severe reactions, with angioedema and systemic symptoms . However, it may be more difficult to identify the causative antigen if this is a contaminant in the food, such as a mould or storage mite, or if there is an unexpected ingredient in the foodstuff and a detailed list of ingredients is lacking.