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Code: f13
Latin name: Arachis hypogaea
Source material: Shelled nuts
Family: Fabaceae (Leguminosae)
Common names: Peanut, Groundnut, Monkeynut

Allergen Exposure

Geographical distribution
Peanuts were first cultivated in South America, as early as 3000 BC. Portuguese explorers transplanted Peanut plants to Africa, and from there they were carried by explorers to the rest of the world.

Peanuts are the seeds of an annual legume, which grows close to the ground and produces its fruit below the soil surface. This is in contrast to tree nuts like Walnuts and Almonds. Peanut is a member of the Fabaceae or legume family, whereas tree nuts are not. The Peanut plant is oval-leafed and about 45 cm tall. Delicate yellow flowers develop around the lower portion. The flowers pollinate themselves and then lose their petals as the fertilised ovary begins to enlarge. The budding ovary or “peg” grows down toward the soil. The Peanut embryo burrows into the soil surface and begins to mature, taking the form of the Peanut.

Multiple Peanut varieties are grown in the USA, with more than 40% of the American Peanut crop consumed as Peanut butter (1). Runners have become the dominant Peanut type grown in the U.S. due to the spectacular increase in yield that they allow; they are a very important source of Peanut butter. Virginias have the largest kernels and account for most of the Peanuts roasted and sold in their shells. Spanish peanuts have smaller kernels covered with a reddish-brown skin. They are used predominantly in Peanut candy, but with significant quantities also used for salted nuts and Peanut butter. They have higher oil content than the other types of Peanuts, which is advantageous for oil extraction. Valencias are small, very sweet Peanuts usually roasted and sold in the shell, or boiled, but seldom used in processed foods.

Peanuts are consumed mainly as Peanut butter, as snacks (roasted, salted, plain or dry roasted), in candy and in baked goods. Peanuts also yield widely used cooking oils (both refined and crude, aromatic and non-aromatic). In China, Peanut is second only to Soy as a source of fat and oil. Peanut flour is an important ingredient in a variety of processed foods. The American George Washington Carver developed more than 300 uses for Peanuts, some of them industrial rather than culinary.

Peanut oil is the refined fixed oil obtained from the seed kernels. Hydrogenated Peanut oil, Peanut acid, and Peanut glycerides are all derived from Peanut oil. The oils and glycerides are skin-conditioning agents in cosmetics. The acid functions as a surfactant or cleansing agent, and the flour is an abrasive, bulking agent and/or viscosity-increasing agent. Peanut oil, if not highly purified (and cold-pressed or extruded oils tend not to be), may contain Peanut allergen (2-4).

Some Peanut-allergic patients react to crude Peanut oil but not refined Peanut oil (5). The quality of Peanut oil in chocolate, for example, may be crucial in determining whether or not allergic reactions occur. Peanut oil has been used to manufacture infant food, resulting in sensitisation in some children as a result of the presence of Peanut allergens (6-7).  Creams and ointments containing Peanut oil may lead to sensitisation; oily solution vitamin preparations are an example (8-9). 

Peanut flour is made from raw Peanuts that have been cleaned, blanched, roasted and processed to produce a lower-fat Peanut flour with a strong roasted Peanut flavour. Peanut flour is used in confectionery, seasoning blends, bakery mixes, frostings, fillings, and health-food bars (10).

Peanuts have been deflavoured, reflavoured and sold as Walnuts, Almonds, and Pecan nuts (11).

Unexpected exposure
See under Environment.

Peanut contains up to 32 different proteins, of which at least 18 have been identified as being capable of binding specific IgE (12-13). Varieties of Peanuts from different parts of the world contain similar proteins, including Ara h I and Ara h 2, and the IgE-binding properties have also been reported to be similar to a great extent (14).

The major Peanut allergens are homologous to the seed storage proteins of the conglutin, vicilin, and glycinin families (15).

Peanut proteins were originally classified as albumins (water-soluble) or globulins (saline-soluble); the globulins were in turn subdivided into arachin and conarachin fractions (the major storage proteins). There are 2 polymorphic forms of arachin, A and B. Conarachin can be separated by ultracentrifugation into 7.8 S and 12.6 S components. The latter component is designated as conarachin II or alpha conarachin. Other components of the albumin fraction of Peanuts are agglutinins, lectin-reactive glycoproteins, protease inhibitors, alpha-amylase inhibitors and phospholipases (16).

The major storage proteins of legumes are globulins, subdivided into legumins and vicillins. The 2 major Peanut allergens, Ara h 1 and Ara h 2, are heat-stable vicillin proteins (17).
A number of allergens have been characterised to date:

  • Ara h 1, a 63.5 kDa – 65 kDa protein, a heat-stable major allergen, and a vicilin seed storage protein. (17-37) Ara h 1 also contains a carbohydrate moiety. (38)
  • Ara h 2, a 17.5 kDa protein, a major allergen, a conglutin seed storage protein and a trypsin inhibitor. (18, 21, 24, 26, 28, 34, 37, 39-52) The isoforms Ara h 2.0101 and Ara h 2.0201 have been isolated. (53-54)
  • Ara h 3, a 60 kDa protein, a major allergen, and an 11S globulin (glycinin-like) seed storage protein. (21, 26, 33, 37, 55-62) Ara h 3 may also function as a trypsin inhibitor. (63-64)
  • Ara h 4, a glycinin seed storage protein. (24, 28, 33) Ara h 4 and Ara h 3 are considered to be the same allergen. (57)
  • Ara h 5, a 15 kDa protein, and a profilin. (24, 65-66)
  • Ara h 6, a 2S albumin, a heat- and digestion-stable protein. (24, 26, 28, 48, 51-52, 67, 68)
  • Ara h 7, a 2S albumin. (24, 28)
  • Ara h 8, a 16.9 kDa protein, a Bet v 1-homologous allergen. (28, 69-71)
  • Ara h 9, a lipid transfer protein. (37, 72-73)
  • Ara h Oleosin, a 18 kDa protein. (74)
  • Ara h Agglutinin. (75-76)

Recombinant proteins include:

  • rAra h 1. (77, 78)
  • rAra h 2. (79)
  • Ara h 3. (57)
  • Ara h 4. (80)
  • Ara h 5. (66)
  • rAra h 6. (67)
  • rAra h 8. (70)

Ara h 1 comprises 12% to 16% of the total protein in Peanut. It has a stable trimeric structure that protects IgE binding epitopes from degradation (21, 81).  In a study where native Ara h 1 from Peanuts was purified using only size exclusion chromatography, the allergen appeared to exist in an oligomeric structure rather than a trimeric structure. As structural characteristics are important for a protein's allergenicity, this may imply a different allergenicity for Ara h 1 than previously described (29).

At least 23 different linear IgE-binding epitopes, located throughout the length of the Ara h 1 protein, have been identified (18). Ara h 1 and Ara h 2 are recognised by 70-90% of patients with Peanut allergy (82-83). However, some subjects fail to bind to either Ara h 1 or Ara h 2 (84). But this may vary among populations: in population studies, Ara h 1 was recognised by over 95% of patients from a North American population, (17) whereas the same allergen was recognised by only 35%, (19) 65%  (24) and 70% (82) of patients of 3 European populations. These differences in recognition were not found for Ara h 2. (14) In some population groups, Ara h2 may be the more prevalent allergen, and previously unidentified Peanut proteins with molecular weights somewhat lower than 15 kDa may be important allergens as well (85).

Although the major allergens are heat-stable and resist gastric acid fluid degradation, and Ara h 2 subjected to roasting has been shown to protect Ara h 1 from proteolytic digestion when co-incubated (39), the allergenicity of Peanut allergens has been clearly shown to be dependant on the degree of heat the substance is exposed to, and it is evident that roasting increases the allergenicity of Peanuts (90, 86-88). For example, the prevalence of Peanut allergy in China is much lower than in the West. The methods of frying or boiling Peanuts practiced in China were shown to result in less allergenicity, compared with the method of dry roasting practiced widely in the United States. Roasting uses higher temperatures (150-170 degrees C) than boiling (100 degrees C) or frying (120 degrees C). This may help explain the difference in prevalence of Peanut allergy observed in the 2 countries. Compared with the amount in roasted Peanuts, the amount of Ara h 1 was reduced in the fried and boiled preparations, resulting in a significant reduction of IgE-binding intensity. There was significantly less IgE binding to Ara h 2 and Ara h 3 in fried and boiled Peanuts, compared with that in roasted Peanuts, even though the protein amounts were similar in all 3 preparations (89). The decrease in allergenicity of boiled compared to roasted Peanut may result mainly from a transfer of low-molecular-weight allergens into the water during cooking (32).

Although native Ara h 1 undergoes a significant heat-induced denaturation on a molecular level, the allergenicity of Ara h 1 is unaffected by heating, indicating that the recognition of conformational epitopes of Ara h 1 by IgE either is not a dominant mechanism or is restricted to parts of the protein that are not sensitive to heat denaturation. (23) Other studies have reported that the protein modifications made by the Maillard reaction contribute to this effect. (90-91)

Furthermore, mature roasted Peanuts have been shown to exhibit a higher IgE binding and advanced glycation end adducts level than immature roasted Peanuts. (91-92)

Other factors may also be involved: for example, the protein concentration is higher in raw Peanuts (approx 16.6 g per 100 g) than in roasted Peanuts (approx 2.6 g per 100 g), and the increased histamine content may have some influence. (93) Ara h 1 levels are up to 22-fold higher in oven-roasted than in raw Peanuts (820 vs. 37 mug/ml. (31) Saliva has been shown to contain up to 1110 mg/ml Ara h 1, and therefore, as calculated through extrapolation, a single kiss could transfer up to 88.8 mg of Peanut proteins in saliva. (94)

In the case of Ara h 2, a protein that functions as a trypsin inhibitor, it was shown that roasting caused a 3.6-fold increase in trypsin inhibitory activity. Functional and structural comparison of the Ara h 2 purified from roasted Peanuts to native and reduced Ara h 2 from raw Peanuts revealed that the roasted Ara h 2 mimics the behaviour of native Ara h 2 in a partially reduced manner. (39)

Ara h 2 consists of 2 isoforms, namely Ara h 2.0101 and Ara h 2.0201. Ara h 2.0201 has similar but higher IgE binding than the originally sequenced Ara h 2.0101 isoform (81% vs 77%) and contains other IgE specificities. (53) Of the 2 Ara h 2 isoforms, Ara h 2.02 might be the more potent allergen. (54) Ara h 2 was found to be a much more potent allergen than Ara h 1. (95) The 2S albumin Ara h 2 is homologous with the minor allergen Ara h 6. Native Ara h 2 and Ara h 6 have virtually identical allergenic potency. The extreme immunological stability of the core structures of Ara h 2 and Ara h 6 provides an explanation for the persistence of the allergenic potency even after food processing. (48)

Children with Peanut allergy recognise mainly Ara h2 and Ara h6, and this recognition remains stable over time. In Peanut-allergic adults, IgE is mainly directed to Ara h1 and Ara h2 (51). 

Ara h 1, Ara h 2, and Ara h 3 are considered to represent >30% of the total protein content of Peanut (96).

Ara h 3 was in the first instance identified as a 14 kDa protein, but cloning of its gene revealed a protein of approximately 60 kDa. (55, 57, 64) Ara h 3 is recognised by serum IgE antibodies from 45% - 50% of patients with Peanut sensitivity (64, 97).  Ara h 3 consists of a series of polypeptides ranging from approximately 14 to 45 kDa that can be classified as acidic and basic subunits; this is similar to the subunit organisation of Soy glycinin (57). Various levels of the 3 major Peanut allergen genes, Ara h 1, Ara h 2 and Ara h 3, and of their corresponding proteins, have been found in all Peanut cultivars. However, Ara h 3 expression patterns among cultivars are more variable than patterns of Ara h 1 and Ara h 2. Transcripts were tissue-specific, observed in seeds but not in leaves, flowers, or roots, and were undetectable during seed germination (98). In a study, 60 accessions in the U.S. Peanut core collection were analysed, along with 88 Florida Peanut breeding program lines. An accession from India had the lowest level of Ara h 1 (7.0%). An accession from Nigeria had the highest level of Ara h 1 (18.5%), but the lowest level of Ara h 2 (6.2%). An accession from Zambia had the highest level of Ara h 2 (13.2%), but the lowest level of Ara h 3 (21.8%). Two accessions, 20 lines, and 2 Peanut cultivars (Florunner and Georgia Red) contained little or no trace of a 36 kDa Ara h 3 isoform, Ara h 3-im (99).

Although Ara h 3 is regarded as a minor allergen, in a study of a group of Peanut-allergic Italian children, it was found that they were specifically sensitised to the basic subunit of Ara h 3 (62).

Ara h 5 shows up to 80% amino acid sequence identity with the panallergen profilin, but Ara h 5 is present only in low amounts in Peanut extracts; (66) 13% to 16% of Peanut-allergic individuals are sensitised to Peanut profilin (66-65).

Ara h 6, which has structural similarities to Ara h 2, also has equal in vitro and in vivo potency (95). Ara h 6 has been reported to be a minor allergen. However, in a study of 29 Peanut-allergic patients, Ara h 6 was recognised by 20. Ara h 6 has homology to Ara h 2, especially in the middle part and at the C-terminal part of the protein; and almost complete inhibition of IgE-Ara h 6 by Ara h 2 demonstrates that at least part of the epitopes of Ara h 6 are cross-reactive with epitopes on Ara h 2; furthermore, Peanut-allergic patients recognise Ara h 6 both in vitro and in vivo to a similar extent as in the case of Ara h 2. Researchers have therefore concluded that Ara h 6 should be considered a major Peanut allergen as well (52).

Compared with Ara h 6, Ara h 2 appears to be the more potent allergen, even though the 2 Peanut allergens share substantial cross-reactivity. Both allergens contain cores that are highly resistant to proteolytic digestion and to temperatures of up to 100 degrees C. The reduction in IgE antibody-binding capacity does not necessarily translate into reduced allergenic potency, and native Ara h 2 and Ara h 6 have virtually identical allergenic potency, compared with the allergens that were treated with digestive enzymes. The folds of the allergenic cores are virtually identical with each other and with the folds of the corresponding regions in the undigested proteins. The extreme immunological stability of the core structures of Ara h 2 and Ara h 6 provides an explanation for the persistence of the allergenic potency even after food processing (48).

In a study of recombinant Ara h 6, the presence of allergen-specific IgE to Ara h 6 was strongly associated with patients having symptoms of anaphylaxis and urticaria, but not with patients having isolated oral allergy syndrome. This may be an indication that Ara h 6 is a candidate for association with severe clinical reactions (100). 

As mentioned above, children with Peanut allergy recognise predominantly Ara h2 and Ara h6, which remains stable over time. In Peanut-allergic adults, IgE antibodies are mainly directed to Ara h 1 and Ara h 2. A role for Ara h 6 in diagnosis has been proposed. In contrast to adults, IgE in children can fluctuate over time, indicating that children may have a more dynamic reactivity to Peanut. In a study that examined the IgE reactivity to major Peanut allergens in Peanut-allergic children at 2 points in time, 20 Dutch children (3-15 years old) with Peanut allergy DBPCFC were evaluated. Before DBPCFC, all Peanut-allergic children showed IgE reactivity to recombinant Ara h 2; Ara h 6 was recognised by 16 children, and Ara h 1 and Ara h 3 by 10 children. After 20 months, Peanut-specific IgE levels (median 23 kUA/l) and the individual recognition of major allergens were comparable with the levels and recognition before challenge (median 28.2 kUA/l). SPT with Ara h 2 and Ara h 6 was positive in most children, whereas SPT for Ara h 1 and Ara h 3 was positive in approximately half of the children. Ara h 6 induced the largest wheals. Therefore, Ara h 2 and Ara h 6 were the most frequently recognised major Peanut allergens in children (101).

Ara h 8 has a low stability to roasting and no stability to gastric digestion. rAra h 8 inhibited IgE binding to Peanut in 4 of 7 tested patient sera (70).

In a study of sera from 12 patients with atopic dermatitis and a positive DBPCFC to Peanut, Peanut agglutinin bound IgE in only 50% of the sera (75). Although Peanut agglutinin is considered in the literature to be a minor allergen, a study reported that the majority of sera from Peanut-sensitive patients showed a high level of IgE binding to the lectin even after heat treatment; and that, contrary to published data, non-enzymatic browning reactions seemed to deteriorate the IgE affinity of the lectin (102).

Nonetheless, individuals are sensitised to a number of allergens in a heterogeneous way, rather than stereotypically to only 1 or 2. For example, in an examination of sera of 40 Peanut-allergic patients, 14 individual recognition patterns and the following frequency of specific IgE binding emerged: Ara h 1 was recognised by 65%, Ara h 2 by 85%, Ara h 4 by 53%, Ara h 5 by 13%, Ara h 6 by 38% and Ara h 7 by 43% of the sera (24).

Similarly, in a study of 30 Peanut-allergic individuals, the majority of patients with a positive SPT were sensitised to Ara h 2 (25/30, 83%) and Ara h 6 (26/30, 87%). Sixteen patients (53%) were sensitised to Ara h 1 and 15 patients (50%) to Ara h 3. All patients with a positive SPT to Ara h 1 and/or Ara h 3 were also sensitised to Ara h 2 and/or Ara h 6. The eliciting dose for subjective reactions varied from 0.1 mg up to 300 mg Peanut, and from 10 to 3000 mg for objective symptoms (95).

Furthermore, a number of uncharacterised allergens have been detected in Peanut. A Peanut agglutinin has been isolated, and bound IgE in 50% of the Peanut challenge-positive patients (75). An oleosin, a member of a family of proteins involved in the formation of oil bodies, has been isolated and found to bind with 3 of 14 sera of Peanut-allergic patients (74). Hybridisation may have no effect on the allergenicity of Peanut: high-oleic Peanuts, known as the SunOleic type, show no difference in allergenicity (88). Similarly, no difference in the allergic components of either raw or roasted extracts of Korean or American Peanuts could be demonstrated (103).

Two different genes encoding class II chitinases have been isolated from Peanut, suggesting that stress to the plant can result in the formation of a chitinase protein (104). The clinical relevance of either gene has not been determined. Similarly, during Peanut maturation and curing, a new class of proteins, namely stress proteins or dehydrin-like proteins, is produced (105).

Peanut allergens have been shown to cross into breast milk and may sensitise infants. In a study of 23 lactating women given 50 g of Peanut to eat, Peanut protein was detected in the breast milk of 11. It was detected in 10 subjects within 2 hours of ingestion and in 1 subject within 6 hours. The median peak Peanut protein concentration in breast milk was 200 ng/ml (range, 120-430 ng/ml). Both major Peanut allergens, Ara h 1 and Ara h 2, were detected. (106) Indeed, Peanut proteins can be found in breast milk for several hours after a mother has eaten Peanuts (107).

Although sensitisation to Ara h 1 and Ara h 2 occurs in the great majority of Peanut-allergic individuals, the wide range of allergens present in whole Peanut protein extract appears to be most appropriate to consider when testing for Peanut allergy (84).

However, appreciating the involvement of individual Peanut allergens may enable better diagnosis and better assessment of severity and prognosis, and may aid immunotherapy.

For example, a Dutch study investigated whether sensitisation to the individual allergens Ara h 1, Ara h 2, Ara h 3 and Ara h 6 correlated with clinical severity. The reactivity of purified Peanut allergens was measured by SPT and by IgE immunoblot in 30 patients. The majority of patients recognised Ara h 2 and Ara h 6. Patients with severe symptoms had a higher skin reactivity to Ara h 2 and Ara h 6 at low concentrations and to Ara h 1 and Ara h 3 at higher concentrations, compared with patients who had mild symptoms. Patients with severe symptoms also recognised a greater number of allergens and showed a higher cumulative SPT response, compared with patients with mild symptoms. No significant differences were observed between patients with a low ED and patients with a high ED. Therefore, Ara h 2 and Ara h 6 appeared to be more potent than Ara h 1 and Ara h 3. Skin reactivity both to low concentrations of Ara h 2 and Ara h 6, and to higher concentrations of Ara h 1 and Ara h 3, was shown to be indicative of severe symptoms. (95) However, in a similar study conducted in the UK to evaluate whether the pattern of IgE binding to specific Peanut allergens is associated with the severity of clinical symptoms, the results demonstrated that the promiscuity of IgE binding appeared more important than the recognition of individual proteins (36).

Potential Cross-reactivity

An extensive cross-reactivity among the different individual species of the Fabaceae (Leguminoseae) could be expected but in fact does not occur frequently(108).  Moreover, the taxonomic classification of Peanut and tree nuts does not appear to predict allergenic cross-reactivity (109).

Although Peanut shares homologous proteins with other beans and legumes, and although several studies have demonstrated that individuals with clinical reactions to a single legume often (38 to 79%) show the presence of IgE sensitisation in SPT and allergen-specific IgE tests to a variety of legumes, the majority do not show clinical reactions to the other legumes. (110, 111, 112) Further evidence indicates that although one would expect Peanut-allergic individuals to have a high risk of cross- or co-reactivity to Soya bean, a family member, blinded food challenges have shown a low rate (110)
Similarly, in a study reporting on reactivity of the sera of Peanut-allergic subjects to 11 different legumes, as shown by in vitro tests, cross-allergenicity was demonstrated to be most marked among the extracts of Peanut, Garden pea, Chick pea, and Soya bean; yet clinical studies found little cross-reactivity among members of the legume family (110). In a study of 60 children with Peanut allergy who were evaluated for allergy to Soy and other legumes (Pea, String bean, Lima bean) by blinded food challenges, 6.5% of those with severe Peanut allergy had reactions to Soy. Overall, only 2 of 41 (5%) with any 1 positive challenge reacted to more than 1 legume. Up to 15% of Peanut-allergic patients may react to other members of the legume family (113). Furthermore, other legumes rarely provoke severe anaphylactic reactions or result in a lifelong allergy (114).

However, patients with Soya bean allergy mainly mediated by cross-reactivity to Birch pollen allergens have been recently described; a majority of whom were reported to have Peanut allergy. Of 9 Swiss and 11 Dutch patients with Peanut and Birch pollen allergy and a positive double-blind, placebo-controlled food challenge result to Peanut, all experienced symptoms in the oral cavity, progressing to more severe symptoms in 40% of patients. Recombinant Ara h 8-specific IgE was demonstrated in 85%, and IgE binding to Ara h 8 was inhibited by Bet v 1 in inhibition studies. In EAST inhibition, recombinant Ara h 8 inhibited IgE binding to Peanut in 4 of 7 tested patient sera. The study concludes that Peanut allergy might be mediated in a subgroup of the patients by means of cross-reaction of Bet v 1 with the homologous Peanut allergen Ara h 8. The study also demonstrated a low stability of Ara h 8 to roasting and no stability to gastric digestion. Basophil histamine release with rAra h 8 was demonstrated in 5 of 7 tested sera (70).

Lupine, which is not a common food in some parts of the world (e.g., USA), is a legume that appears to have a high degree of cross-reactivity with Peanut. In a French study, 11 of 24 Peanut-allergic individuals were shown to have skin reactivity to Lupine flour. Double blind oral challenges with Peanut and Lupine flour performed in 8 of the 24 patients were positive in 7 when the same dose of both was used. The major Lupine flour allergen (a 43 kDa protein) is present in Peanuts. The risk of cross Peanut-Lupine allergy was reported to be high, in contrast to the risk with other legumes. The authors suggested that the inclusion of 10% Lupine flour in Wheat flour without mandatory labelling makes Lupine a hidden allergen, presenting a major risk of cross-reaction in subjects already allergic to Peanut products (115). Further confirmation of the high risk of cross-reactivity between Lupine and Peanut is documented by a case of Peanut cross-allergy to Lupine flour in hot dog bread; (116) there is also reported to be an increased risk of serious acute asthma due to Lupine flour, a risk associated with Peanut allergy (117).

Reactivity to tree nuts is a serious problem for Peanut-allergic people. Peanut and tree nut allergic reactions coexist in 25-50% of Peanut-allergic patients, and allergic reaction to tree nuts such as Walnuts, Cashews, Pecans and Pistachios can develop even though tree nuts are from a different botanical family (118-120). Reactions frequently occur on first known exposure and may be life-threatening. It is unclear whether this is due to genuine cross-reactivity or to the coexistence of separate allergies in broadly atopic individuals. Patterns of reported co-sensitisation vary among studies. In 60 adult Peanut-allergic individuals, 40 (66%) reported allergy to tree nuts, but 50 (83%) had a positive SPT to 1 or more tree nuts (5) . The relative frequency of allergy to individual tree nuts concomitant with Peanut allergies is reported to reflect the consumption of various types of nuts by the general population: Almond, Hazel and Brazil nuts are implicated far more commonly than Pecan, Cashew and Pistachio (165).

Or course, cross-reactivity can be calculated from the other direction. For example, 5 of 12 Brazil nut-allergic individuals were also allergic to Peanut (121).

Ara h 1 has a high sequence similarity with other plant vicilins, members of the cupin superfamily (122). However, although there are significant areas of homology between Ara h 1 and Soy vicillins, these areas do not appear to be significantly involved in the binding of IgE antibodies, which may account for the low frequency of coexistent Peanut and Soy allergy, despite the frequency of positive SPT to Soy(55, 78). Similarly, even though Jug r 2, the allergen from Walnut, exhibited significant homology to the vicilin group of seed proteins, including those in Cocoa bean (Theobroma cacao) and Cottonseed (Gossypium hirsutum), there is minimal or no cross-reactivity between Jug r 2 and Pea vicilin, Peanut proteins, or Cacao proteins (123). Nonetheless, this may not be completely clear-cut, as demonstrated by a report on 3 patients with a history of anaphylaxis to Pea who subsequently had symptoms after ingestion of Peanut. Peanut-related symptoms consisted of oral symptoms in all patients, with additional urticaria and dyspnoea or angioedema in 2 patients. All patients had a positive skin prick test response and an increased IgE antibody level to Pea and Peanut. Immunoblotting revealed strong IgE binding, mainly to vicilin in Pea extract and exclusively to Ara h 1 in crude Peanut extract. IgE binding to Peanut could be inhibited by Pea but not, or only partially, the other way around. Clinically relevant cross-reactivity between Pea and Peanut does occur and is attributable to vicilin homologues (124).

The major Peanut allergen Ara h 2 shares IgE-binding epitopes with Almond and Brazil nut allergens, which may contribute to the high incidence of tree nut sensitisation in Peanut-allergic individuals (125).  However, an earlier study reported that conformational analysis of the linear IgE-binding epitopes mapped on the molecular surface of Ara h 2 showed no structural homology with the corresponding regions of Walnut Jug r 1, Pecan Car i 1 or Brazil nut Ber e 1; it was inferred that the absence of epitopic community does not confirm the allergenic cross-reactivity observed between Peanut and Walnut or Brazil nut, which presumably depends on other ubiquitous seed storage protein allergens, namely the vicilins. However, the major IgE-binding epitope identified on the molecular surface of the Walnut Jug r 1 allergen shared a pronounced structural homology with the corresponding region of the Pecan nut Car i 1 allergen. With the exception of Peanut, 2S albumins could thus account for the IgE-binding cross-reactivity observed among some other nuts, e.g., Walnut and Pecan nut (49).

Ara h 3, an 11S globulin seed storage protein family member, may result in cross-reactivity between Peanut other foods containing other such family members, including Hazelnut and Soya bean. Homology among these 3 proteins ranges from 45% to 50%. One IgE binding epitope of Ara h 3 has a 67% homology of amino acid residues with the corresponding area of Cor a 9 of Hazelnut (56).

Ara h 5 shows up to 80% amino acid sequence identity with the panallergen profilin, but this allergen is present only in low amounts in Peanut extracts. Immunoblot analysis of 50 sera from individuals sensitised to Peanut showed that 16% had mounted a detectable IgE response to the newly identified Peanut profilin, which indicates some risk for cross-reactivity with other profilin-containing foods and plants (66).

Ara h 8 also has cross-reactive potential. Examination of the sera of 5 patients in relation to 4 recombinant allergens led to the conclusion that IgE cross-reactivity existed between Bet v 1 and its homologues Gly m 4 from Soybean, Ara h 8, and Pru av 1 from Cherry. On all 4 proteins, 1 IgE-binding surface area that was recognised by all patients, and 2 that were recognised by 3 patients, were identified (126). Lupine is an emerging cause of food allergy because of recent large-scale introduction into processed foods and frequent cross-reactions with other members of the legume family. Sequence comparison and modelling demonstrated highly significant sequence homology and molecular similarity between the allergen Ara h 8 of Peanut and the pathogenesis-related protein PR-10 of White lupine. (Another protein of Lupine, the beta-conglutin precursor, was found to be significantly homologous to the Ara h 1 allergen of Peanut) (127).

In a study, approximately a third of patients sensitised to grass pollen were found, in specific IgE investigations, to have significant serum levels of anti-Peanut IgE, without positive Peanut skin reactivity and without Peanut-related allergic symptoms. This was attributed to the presence of cross-reactive carbohydrate determinants (CCD) of glycoproteins. In a study investigating the biologic activity of IgE directed to CCD, cross-reactive IgE directed to carbohydrate determinants of glycoproteins, as found in grass pollen-sensitised patients, was shown to have poor biologic activity, therefore causing positive RAST results without apparent clinical significance (38).

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Clinical Experience

IgE-mediated reactions
Peanuts are a significant cause of serious food allergy in both adults and children. Unlike other food allergies, Peanut allergy usually begins in childhood and persists throughout the affected individual’s lifetime. Only approximately 20% of young children will develop tolerance (112, 128). More precisely, Peanut allergy persists in about 80% of young children reacting to Peanut in the first 2 years of life, and virtually all individuals reacting to Peanut beyond the age of 5 years. (129 )Approximately 75% of children experience a reaction with their first known Peanut exposure suggesting, early sensitisation (120, 130-131.). Peanut allergic reactions are more likely to be severe or even fatal than reactions to other food allergens (13).

Atopic dermatitis, angioedema, asthma, diarrhoea, nausea and vomiting, and anaphylaxis have been reported. Urticaria may be a prominent symptom (132). Angioedema of the lips and tongue following ingestion of Peanut butter, and localised urticarial reactions following direct skin contact, were described (133). A patient with a history of a burning tongue, together with discomfort of the labial and buccal mucosae, improved after Peanut was removed from her diet (134). Severe reactions may be associated with abdominal pain at the onset of the response (17, 43). In one study, the commonest symptom was facial angioedema, and the major feature accounting for life-threatening reactions was laryngeal oedema (119). Food-dependant exercise-induced anaphylaxis has been described (135-137). A study reported a high frequency (50%) of food hypersensitivity in patients with allergic rhinoconjunctivitis; Peanut was one of the most frequent food allergens encountered (138). A significant association was reported to exist between recurrent serous otitis media and food allergy in 81 of 104 patients. An elimination diet resulted in a significant amelioration of the disease in 86% of the patients, and a challenge diet provoked recurrence of symptoms in 94%. The most prevalent allergens involved were Cow’s milk, Wheat, Hen’s egg, Peanut, Soy and Corn (139).
Although not reported frequently, asthma may be a significant feature in Peanut allergy. In a French study evaluating 163 asthmatic children with food allergy, and exploring food-induced asthma via double-blind placebo controlled food challenges, the most frequent offending foods were found to be (sometimes in association) Peanut (30.6%), Hen’s egg (23.1%), Cow's milk (9.3%), Mustard (6.9%), Codfish (6%), Shrimp (4.5%), Kiwi fruit (3.6%), Hazelnut (2.7%), Cashew nut (2.1%), Almond (1.5%), and Garlic (1.2%). (140) Asthma caused by Peanuts in a 3-year-old child was established by chance, and strict avoidance of Peanut led to complete remission of symptoms and rapid termination of inhalation therapy. The authors suggested that as severe Peanut allergy in asthmatic infants carries a risk of anaphylaxis, it is useful to look for Peanut allergy in all infants with severe asthma (141).

Food allergy affects 6-8% of children < 4 yrs of age in the USA, and 2% of the population >10 years of age. It is the leading cause of anaphylaxis treated in hospital casualty departments (114). Food allergy accounts for about 30,000 anaphylactic reactions, 2,000 hospitalisations and 200 deaths each year in the USA (142). Allergy to Peanuts and tree nuts accounts for the majority of fatal and near-fatal anaphylactic reactions (143-144). About a third of Peanut-sensitive patients have severe reactions to Peanuts. As little as 100 micrograms of Peanut protein provoke symptoms in some subjects with Peanut allergy. Asthmatics with Peanut sensitivity appear more likely to develop fatal reactions, and this is thought to be because of the exquisite sensitivity that asthmatics have to the chemical mediators of anaphylaxis. Severe reactions may occur within a few minutes of ingestion. Peanut may in particular affect children with chronic severe asthma. In a double-blind food challenge of 38 children, Peanut resulted in adverse reactions, which were chiefly gastrointestinal, even though asthma was the common presenting complaint (145).

In the USA, a national survey indicated that about 0.8% of children reported allergy to Peanut. (146) Other studies concur, showing that almost 1 in 150 children has a Peanut and/or tree nut allergy (147). In one registry of patients with Peanut allergy, more than 70% had had their first allergic reaction after their first apparent contact with Peanuts. Since reactions require previous exposure for sensitisation, and since IgE antibodies do not cross the placenta, these findings suggest that Peanut protein was encountered in utero, or through breast milk. (106, 148 )In a French study of 54 infants who were less than 11 days old and 71 who were 17 days to 4 months old, 8% had a positive skin-prick test for Peanut (149). Approximately a third of American children with moderate to severe atopic dermatitis and food hypersensitivity are allergic to Peanuts (150).

Peanut allergy is self-reported by 1 in 200 of the British population (151). According to a population-based study of 3- to 4-year-olds in the United Kingdom, the prevalence of sensitisation to Peanuts increased 3-fold from 1.1% to 3.3% from 1994 to 1996. Of 41 sensitised children in a study, 10 reported a convincing clinical reaction to Peanut, and 8 had positive oral challenge results, giving an overall estimate of Peanut allergy of 1.5% (18/1246) (152). Serological evidence of sensitivity to Peanuts from data gathered from 1988 to 1994 indicated that about 6% of Americans had IgE antibodies to Peanut (although the majority would not have an allergic reaction when eating Peanuts) (153).

The prevalence of Peanut allergy in a study of 7768 primary school children in Montreal, Canada, was 1.50%. When multiple imputations were used to incorporate data on those responding to the questionnaire but withdrawing before testing, the estimated prevalence increased to 1.76%. When data regarding the Peanut allergy status of non-responders were also incorporated, the estimated prevalence was 1.34% (154).

In a group of 580 patients in France with reactions to food, 60 presenting with severe, near-fatal reactions, 37% were sensitised to Peanut (155). A second French study of food allergy in 544 children aged 0 to 15 years, who were evaluated with oral food challenges, found that 24% had Peanut allergy and 4% had a tree nut allergy. Clinical symptoms attributed to Peanut allergy included atopic dermatitis (46%), urticaria/angioedema (32%), asthma (15%), generalised anaphylaxis (5%), and gastrointestinal symptoms (3%) (156).

There is good evidence that Peanut allergy in on the increase. (146, 152, 157). When and how does sensitisation occur, and why is it increasing? Several possibilities have been investigated. A factor postulated to have contributed to Peanut allergy in the UK is the cutaneous exposure to ultra-low doses of Peanut antigens in Peanut oil found in diaper rash emollients, which are applied to the skin of infants with eczema or diaper rash (147, 158). Differences in the way Peanuts are prepared may contribute to the variations in prevalence. For example, the per capita consumption of Peanuts in China is similar to that of the United States; yet Peanut allergy is very rare in China (114).

First sensitisation has been attributed to the presence of Peanut allergen in breast milk. In a report on 8 infants with immediate hypersensitivity reactions to foods, including Peanut, occurring at the first-known exposure, the most likely route of sensitisation was thought to be breast milk, and reactions were thought to be dose-dependent. Symptoms experienced included irritability, erythematous rash, urticaria, angioedema, vomiting, rhinorrhoea, and cough (159).

Peanut allergy has also been reported to be transferred through liver, kidney, (160-161) and bone marrow transplantation (162).

Highly processed oils (acid-extracted, heat-distilled) do not contain Peanut protein (5) .However, cold-pressed or extruded Peanut oils contain Peanut protein. This may result in adverse reactions, such as from Peanut oil-based vitamin preparations (8) and infant milk formulae. (6) In a study of adverse reactions to crude Peanut oil, 10% of Peanut-allergic subjects reported allergic reactions. Refined Peanut oil did not pose a risk to any of the subjects (5).

Peanut may be a hidden allergen, with very serious consequences. For example, fatal anaphylaxis to ingestion of undeclared Peanut flour was reported (163). The problem is especially severe in that Peanut can result in severe reactions to even minute amounts, and even through skin contact. (130, 164)

Infants have reacted adversely even to breast feeds after maternal consumption of Peanut (165). However, the risk of adverse reactions may be product-specific: casual exposure to Peanut butter is unlikely to elicit significant allergic reactions. The authors, however, warn that the results cannot be generalised to larger exposures or to contact with Peanut in other forms (flour and roasted Peanuts) (166).

But extreme cases should be kept in mind. A 9-month-old male developed localised urticaria when his mother kissed him after she had eaten cereal with milk, and generalised urticaria manifested after his brother ate a Peanut butter sandwich and barely touched his bare leg. His first dose of Egg white as half a teaspoon of meringue caused generalised urticaria and conjunctivitis. Approximately an eighth of a slice of bread caused a similar reaction. Similarly, a 5-year-old developed wheezing when entering a classroom of a teacher who had just eaten Peanuts (167). Peanut allergens may be transferred on playing cards (168). As the fat content of a challenge vehicle has been shown to have a profound effect on the reaction experienced after allergen ingestion, fat content may need to be considered in assessing the risk of certain foods to food-allergic individuals (169).

Reactivity to tree nuts is a serious problem for Peanut-allergic people. Peanut and tree nut allergic reactions coexist in 25-50% of Peanut-allergic patients (118-120). In a study of 62 patients with nut allergy (adults and children), Peanuts were the commonest cause of allergy (47), followed by Brazil nuts (18), Almonds (14), and Hazelnuts (13). (119) Similarly, in a study of 122 children attending a food allergy clinic, 68 experienced acute reactions to Peanut alone, 20 to tree nuts but not Peanut, and 34 to both Peanut and at least 1 tree nut. Of the total of 54 children with reactions to tree nuts, 34 had reactions to 1 kind of tree nut, and 20 had reactions to 2 or more different tree nuts, the most common being Walnut, Almond, and Pecan. First reactions usually occurred at home, and at a median age of 24 months for Peanut and 62 months for tree nuts. The skin was the most common organ affected (89% of reactions, 39% as the only system involved), but the respiratory (52%) and gastrointestinal tract (32%) were also affected (118).

Compiled by Dr Harris Steinman


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As in all diagnostic testing, the diagnosis is made by the physican based on both test results and the patient history.