Allergen Profile, Peanut, IgE With Component Reflexes*

Allergen-specific IgE assays do not demonstrate absolute positive and negative predictive values for allergic disease. Clinical history must be incorporated into the diagnostic determination. Although the use of component resolved IgE testing may enhance the evaluation of potentially allergic individuals over the use of whole extracts alone, it cannot yet replace clinical history and oral food challenge in most cases. Sensitization against thus far unidentified determinants that are not found in the whole extract our in components might cause symptoms in rare cases.

Peanut

While sensitization to peanut is common, with somewhere between 8% and 12% of the general public showing sensitivity on either skin prick test or peanut extract allergen specific IgE testing, the majority of these people are not allergic.^6-11

Peanut Components

Ara h 1

Ara h 2

Ara h 3

• Sensitization to Ara h 1, 2 and 3 is usually acquired in childhood.¹⁶

• IgE to Ara h 2 has the best discriminative ability of all diagnostic tests for peanut allergy. It can accurately diagnose peanut allergy in 28% of patients but cannot be used to exclude a peanut allergy in an adult population.^6,12-16

• The presence of antibodies to Ara h 1 and/or Ara h 3 increases the risk of severe reactions.^{8,12,14,13,17-24}

• These three major peanut allergens (Ara h 1, Ara h 2 and Ara h 3) contain similar peptide sequences accounting for the high extent of cross-reactivity observed among them.²⁵

• Ara h 1, Ara h 2 and Ara h 3 have been associated with severe symptoms, although anaphylactic reactions have been described in patients negative for these allergens.⁶

• Reactivity to these dominant allergens at the time of diagnosis in a population of North American infants and toddlers with early-onset peanut allergy was found to be associated with future allergy persistence at age 13 years.²⁶

^{Ara h 6}

• Ara h 6 is a major peanut allergen showing similarity with Ara h 2 in many aspects.^37-41

• Because both are storage proteins of the 2S albumin type that are heat stable and resistant to digestion in the gut, they are associated with potentially systemic reactions.^38,39

• Up to 4 out of 100 peanut allergic patients are monosensitized** to Ara h 6.^40,41

^{Ara h 8}

• IgE antibodies to the Ara h 8 are seldom associated with systemic reactions but more often to local reactions like oral allergy syndrome.^16,21,27-31

• Birch-sensitized individuals are frequently co-sensitized to peanut Ara h 8.³¹

• Sensitization to this birch pollen cross-reactive allergen varies depending on local exposure to birch pollen.^21,29,31

• Children that are mono-sensitized to Ara h 8 can usually safely ingest peanut.²⁸

^{Ara h 9}

• Ara h 9 is a minor component of peanuts.³⁴ However, sensitization to this type of allergenic protein, referred to as a Lipid Transfer Protein, is generally associated with severe reactions in addition to OAS.^32-35

• Ara h 9 has been correlated with mild to severe symptoms in Mediterranean patients (Vareeda, Ballmer-Weber).

General Comments

• The heterogeneity in the clinical and immunological phenotype of peanut allergy in distinct geographical areas reflect exposures to different environmental pollen and differences in dietary traditions.^16,36

• The most commonly occurring sensitization to peanut proteins in individuals who tolerate peanut is directed to Ara h 8 and Ara h 9.¹⁶

• Use of component testing allows for the assessment of sensitization to proteins that may be under-represented in whole peanut extracts.³⁶