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Code: t4
Latin name: Corylus avellana
Source material: Pollen
Family: Corylaceae

See also: Hazel nut f17

Allergen Exposure

The hazel tree is native to all of the British Isles and Europe, and to West Asia and North Africa. It has been naturalised in other parts of the world.

The hazel is a deciduous tree growing to 6 m, but some cultivars are small shrubs or grow in hedges. The tree often develops numerous upright stems from the roots, creating a thicket. The bark is pale brown or grey-brown, and smooth on older stems. The 7.5-10 cm-long green leaves mature to a dark green, becoming reddish in autumn.

The hazel tree is monoecious (individual flowers are either male or female, but both sexes can be found on the same plant). The male catkins open in late winter or early spring in bunches of bright yellow, drooping ‘lamb tails’. The female flowers are almost inconspicuous and appear on the same branches, as tiny pink-red tufts. The tree is wind-pollinated. The fertilised flowers develop into clusters of 2 cm nuts, which turn brown in October.

The tree’s natural habitat is woods and hedgerows, and particularly the slopes of hills.

The tree produces the well-known hazelnut.

Allergen Description

Hazel tree pollen contains at least 40 distinct antigens. (1, 2)

The following allergens have been characterised: 

Cor a 1, a 17.4 kDa protein, a Bet v 1 homologue (a Group 1 Fagales-related protein). (3, 4, 5, 6, 7, 8, 9, 10)

Cor a 2, a 14 kDa protein, a profilin. (3, 4, 8, 11, 12, 13)

Cor a 10, a 70 kDa protein, a luminal-binding protein. (3, 14, 15)

Cor a 1 and Cor a 2 are found in both hazel pollen and hazelnut. (9)

A study evaluating serum IgE of 25 patients with type I allergic reactions to tree pollens and intolerance to hazelnuts reported that all sera bound to Cor a 1 and 16% bound to Cor a 2, the hazel pollen profilin. Similar proteins were found in hazelnuts. (9)

The major allergen Cor a 1, a 17-kDa protein, has been isolated from hazel pollen. Four Cor a 1 isoforms have been determined, which display different antigenic and allergenic properties. The sequence identities between these isoforms and Bet v I, the major birch pollen allergen, were shown to have 80.5-83% similarity. (6)

Potential Cross-Reactivity

Extensive cross-reactivity between the different individual species of the genus could be expected. (16)

Cor a 1 has a 80.5-83% similarity to allergens of other trees of the order Fagales: Bet v 1, the major birch pollen allergen; Aln g 1, the major allergen from grey alder tree; and Car b 1, the major allergen from hornbeam tree. (1) Approximately 95% of patients allergic to tree pollens from the order Fagales display IgE binding to these allergens. (1, 7, 17, 18, 19)

Further evidence for clinical cross-reactivity in this family is the finding that the allergens of birch (Bet v 1), alder (Aln g 1), hazel (Cor a 1), and hornbeam (Car b 1) are confined to shared epitopes. (20)

Cross-reactivity has also been demonstrated between members of the Fagales family (birch, alder, hazel, and beech) and with members of the Oleaceae family (ash, olive, lilac, and privet). (21)

As an antigenic relationship between hazel pollen and hazelnut exists, cross-reactivity can be expected. (22) This relationship has been clarified by a study in which specific IgE from 25 patients displaying type I allergic reactions to tree pollen and intolerance to hazelnuts bound to Cor a 1 in all of the subjects, and to the 14 kDa hazel pollen profilin in 16%. IgE binding to proteins of comparable molecular weights in hazelnut extracts (18 kDa and 14 kDa) was found, suggesting that proteins similar to Cor a I and hazel profilin might also be expressed in hazelnuts. (9)

Cro s 2, a profilin from saffron, has a high degree of amino acid identity and similarity to other plant profilins, including with Cor a 2 from hazel tree (79%). (23) However, the degree of cross-reactivity between profilins may vary considerably between different profilin allergens.  (13)

Clinical Experience

IgE mediated reactions

Hazel pollen is important, resulting in asthma, hay fever and allergic rhinitis in hazel pollen-allergic individuals, in particular in early spring. (19, 24, 25, 26)

Hazel pollen has been shown to be an important cause of pollinosis in Switzerland, (27, 28) Sweden, (29) Germany, (30) Romania, (31) and Italy. (32) High hazelnut pollen levels have been recorded during March in Siena, in central Italy, (33) and in Rabka, Poland. (34) In Jena, Germany, specific IgE tests for 10 tree pollens demonstrated that hazel tree pollen was among the most important causes of spring hay fever. (35)

In Genoa, a northern Mediterranean area in Italy, of 3 473 patients suffering from seasonal respiratory allergy who were SPT to 1 or more pollens, 16% were STP-positive to Betulaceae and/or Corylaceae pollens. These patients had winter or early spring hay fever, with a high incidence of asthma. (36)

A study determined the impact of different trees on asthma and explored the association between daily hospitalisations for asthma and daily concentrations of different tree pollens in 10 large Canadian cities. In correlating interquartile increases in daily tree pollen concentration with percentage increases in daily hospitalisation for asthma, researchers observed a statistically significant but small (<2%) effect for Corylus. (37)

Hazelnut pollen has been detected in aerobiological studies in Zagreb, Croatia; (38, 39) Lublin, Poland; Skien, Norway; (40) and Crete (41) and Thessaloniki, Greece. (42)

In a study of 5 080 asthmatic children aged 1 to 18 years evaluated at an allergy clinic in Istanbul, 6% (345 cases) were sensitised to hazelnut pollen. (43)

A French study suggests that Hazel pollen allergy may result in aggravation of winter respiratory symptoms. A group of patients is described who experienced exacerbation of allergic symptoms due to alder pollen and hazel pollen, but not to infectious coryza. (44)

Other reactions

A 62-year-old Turkish farmwoman, who had been collecting large amounts of green and brown hazelnut leaves for use as fuel, was reported to have developed cough, respiratory distress and intermittent fever. The authors state that continual antigen exposure to mould from hazelnut husks resulted in allergic alveolitis. (45)

Compiled by Dr Harris Steinman, harris@allergyadvisor.com


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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.