Peanut allergy is one of the most common food allergies and, in the Western world, is thought to be prevalent in up to 1 in 200 individuals in some populations. In fact, studies have suggested that peanut allergy in American children younger than five years doubled in prevalence between 1998 and 2003. In the United Kingdom, around 0.4 to 0.6% of the population suffers from peanut allergies.

Studies have shown that peanut allergy makes up about 28% of all food allergies in children and it develops before the child reaches their first birthday in around half of cases. The allergy rarely develops in children older than 15 years.

Symptoms

Some of the symptoms of peanut allergy include:

• A red, raised, itchy rash called urticaria
• Swelling of the face, lips, eyes, mouth and throat, called angioedema
• Constriction of the airways leading to tightness of the chest, wheezing and flare-ups of asthma
• Runny nose, watering and red eyes (allergic conjunctivitis) and nasal congestion.
• Nausea, vomiting and diarrhea
• Peanut-containing oils, creams, lotions or cosmetics may cause contact dermatitis and eczema.

Anaphylaxis

Anaphylaxis is a very serious and life threatening allergic reaction that leads to a severe drop in blood pressure, constriction of the airways and swelling of the larynx that can cause choking. Urticaria and angioedema may be also be present.

Mechanism of peanut allergy

Around 70% of children with peanut allergy develop the allergy on their first known exposure to peanuts. However, this has usually been preceded by an unknown exposure to the nut when no noticeable symptoms were triggered but the child became sensitized or primed for allergy.

On this original, first exposure, the immune system reacts to proteins in the peanuts by producing the antibody immunoglobulin E (IgE) which binds to receptors present on mast cells and basophils. Subsequent exposure to peanut leads to an inflammatory response governed by these cells, as the peanut protein causes the IgE/receptor complexes to cross link and activate the release of the inflammatory mediators (eg histamine) inside them. Histamine and other inflammatory mediators then trigger the symptoms of an allergic reaction.

Examples of unnoticed first exposure to the peanut protein include:

• Exposure of fetus to peanut protein consumed by the mother
• The presence of the allergen in the mother's breast milk
• Exposure to the peanut extract found in oils and lotions worn by other individuals

Diagnosis and treatment

• Diagnosis of peanut allergy involves a detailed evaluation of a patient's diet history and the severity and duration of their symptoms on exposure to peanuts. Next, a skin test for the allergy may be performed or a blood sample taken to check for IgE. In rare cases, a positive challenge test may be performed, where the peanut protein is administered to the patient to examine his or her allergic reaction.
• The most common approach to managing peanut allergy is complete avoidance of the peanut protein. This can be difficult to achieve and requires constant vigilance with regard to reading the labels of any products that may contain nuts.