Written by epgonline.org - Last updated 29 May 2018
Allergy is a broad subject that covers a range of diseases. In general terms, an allergy is an immune reaction to an environmental substance that would not provoke a reaction in most people – these are typically pollens, foods, insect stings and metals. The severity of reaction can vary significantly, ranging from a mild irritation to life-threatening anaphylaxis requiring urgent treatment.
A typical cutaneous reaction provoked either topically or systemically is a wheal or hive. Usually surrounded with erythema, it is a raised, oedematous skin lesion partially driven by histamine reaction. Wheals may be confined to a particular area, or may be more generalised as urticaria. Urticaria can be a self-limiting condition, particularly if a trigger is identified and removable, but in some cases it can become chronic and cause ongoing symptoms and distress. Treatment usually includes high dose antihistamines; more information on the chronic form of spontaneous urticaria (CSU) and its treatment can be found in the CSU knowledge centre.
Another common manifestation of an allergy is dermatitis – a broad term for inflammation of the skin – and presents with pruritus, erythema and a variable rash. Eczema is often difficult to distinguish from dermatitis clinically; atopic eczema is included within Allergy, reflecting its prevalence among people sensitive to environmental factors. Key treatments are emollients and topical steroids, with a potential role for imunosuppressants in more extreme cases.
Anaphylaxis is the most severe and immediately life-threatening consequence of allergy, and is characterised by a rapid onset of widespread rash, angioedema, vomiting, distributive shock and airway compromise. If untreated it will often result in death. Treatment is with a combination of fluids for circulatory support, steroids, IV antihistamines and adrenalin (epinephrine). Patients with a history of anaphylaxis may be provided with a self-injecting adrenaline dose to use at onset of anaphylaxis.
Other manifestations of allergy include allergic rhinitis, food allergy – which may present in numerous ways – as well as rarer conditions such as allergic bronchopulmonary aspergillosis. The latter is a hypersensitivity reaction to aspergillus in the airways, resulting in inflammation and potentially long-term scarring resulting in bronchiectasis.
Allergy as an entity is rising in prevalence worldwide; an existing hypothesis suggests that lack of early exposure to some substances (due to allergen avoidance, increased hygiene awareness and antibacterial cleaning products in modern society), may lead to an enhanced reaction when the stimulus is eventually encountered. Some recent studies have demonstrated a reduced susceptibility to developing nut allergy when high-risk children are exposed to peanuts from a young age.