Author: Bethany Frances Ferris, Foundation Year 2 Doctor, Isle of Man, United Kingdom. DermNet NZ Editor in Chief: A/Prof Amanda Oakley, Dermatologist, Hamilton, New Zealand. Copy edited by Gus Mitchell. February 2019.
Peanut allergy is an adverse immune response to a peanut allergen. Reactions include:
Peanut allergy is the most common cause of food-related anaphylaxis .
The peanut (Arachis hypogaea) belongs to the legume family and is distinct from the tree nut family.
There are 11 peanut allergens (Ara h 1 to Ara h 11) which are seed storage proteins; these are biological reserves that enable the seed to grow into a plant.
The prevalence of peanut allergy in the United Kingdom is reported to be in 0.2–2.5% of children and in 0.3–0.5% of adults [4,5]. A rise in the prevalence was reported in the USA, with 1.4% of children with peanut allergy in 2008 compared to 0.4% in 1997 .
There is a greater risk of peanut allergy in children who have:
Having a peanut allergy does not increase the risk of allergy to another legume (eg, peas, beans, lentils, soybeans) with the exception of lupin. Yet, one-third of those with a peanut allergy will have a concurrent reaction to a tree nut (eg, walnut, almond, brazil nut, coconut) .
Peanut allergy is more prevalent in the Western world than in China, possibly due to the greater consumption of roasted peanuts rather than raw peanuts .
The Learning Early About Peanut Allergy (LEAP) study found that high-risk infants with eczema or egg allergy were less likely to develop a peanut allergy if they had early and sustained consumption of peanuts .
Non-allergic mothers are now encouraged to eat potentially allergenic foods such as peanuts regularly during pregnancy and not to delay introducing their babies to them .
The cause of peanut allergy is not fully understood.
To develop a peanut allergy, the individual must be exposed to one of the peanut allergens via a gastrointestinal, cutaneous, or respiratory route.
Anaphylaxis causes dyspnoea and wheeze (due to bronchospasm and laryngeal oedema), tachycardia, hypotension, dizziness and loss of consciousness. Anaphylaxis is a life-threatening clinical emergency .
A late phase allergic reaction can develop 2–6 hours after the initial exposure to the allergen and peaks at around 6–9 hours. This is due to the recruitment of leukocytes and antigen-specific T cells. The late phase reaction results in erythema and oedema, sneezing, itching, and coughing. It usually fully resolves in 1–2 days [12,13].
Children with asthma have higher mortality from peanut-induced anaphylaxis than non-asthmatic children .
Peanut allergy is principally a clinical diagnosis based on the rapid development of allergic symptoms and signs after eating a peanut.
Skin prick testing and serum specific IgE tests are used to identify sensitisation and to confirm the diagnosis .
Skin prick testing involves placing a drop of peanut allergen on the skin, then pricking to see if a weal is produced within 15 minutes. The British Society of Allergy and Clinical Immunology (BSACI) states that a weal ≥ 8 mm is highly predictive of peanut allergy. Skin prick testing must be performed in a specialist centre with emergency equipment available in case of anaphylaxis .
Serum specific IgE testing, also known as radioallergosorbent testing (RAST), is performed to detect allergen-specific IgE in the blood. Specific IgE ≥15 kU/L is highly predictive of peanut allergy .
The tests do not predict the severity of clinical allergy .
An IgE-mediated type 1 hypersensitivity reaction and ensuing anaphylaxis can be due to another cause. For example:
The sudden development of a rash might be due to the non-allergic release of histamine, as in scombroid fish poisoning.
The treatment of anaphylaxis is a medical emergency with stabilisation of airway, breathing, and circulation.
Confirmed peanut allergy needs a comprehensive management plan, which should be shared with the wider family and school [4,15,16].
Ingesting or touching peanuts, peanut butter, peanut flour, arachis oil and other peanut-containing products must be completely avoided. Read ingredient lists and warnings on manufactured food (in New Zealand, the USA, and many other countries, the possibility of an item containing peanuts must be declared on the packet). Be particularly careful when eating away from home, where unintended contamination of other foods with peanuts may occur.
It is unclear whether patients with peanut allergy should also avoid all legumes and tree-nuts.
Antihistamines should be carried at all times and taken if an allergic reaction occurs. The patient and their carers should be regularly trained in how to use an adrenaline auto-injector or adrenaline in a prepared syringe, and if the device has to be used, they must seek immediate medical attention .
Between 5 and 9% of siblings of children with a peanut allergy will also have a peanut allergy. In high-risk individuals (with asthma, eczema or other food allergies) or in cases of parental anxiety, it is advisable to perform skin prick testing or specific IgE before the child introduces peanut into their diet. In low-risk individuals, peanuts can be carefully introduced to test for an allergic reaction .
Clinical trials of oral, sublingual and epicutaneous peanut immunotherapy have shown some promising results, but at present, are not routinely offered as a treatment for peanut allergy .
Humanised anti-IgE monoclonal antibody therapy using omalizumab has been shown to speed up desensitisation in peanut immunotherapy .
About 20% of children with peanut allergy will grow out of peanut allergy . The allergy persists into adult life in the majority of affected individuals.
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