Latin name: Fraxinus excelsior
Source material: Pollen
Common names: European ash, Common ash, European common ash
See also: White ash (Fraxinus americana) t15.
There are 4 important genera in the Oleaceae family: olive (Olea), ash (Fraxinus), lilac (Syringa), and privet (Ligustrum).
Fraxinus is a genus of about 65 species, which are distributed in northern, temperate regions. The hardy white ash (Fraxinus americana), for example, is native and common in eastern North America and also thrives in Europe. The European ash (Fraxinus excelsior) is native to most of Europe, with the exception of northern Scandinavia and the southern Mediterranean. White ash tree (t15) needs to be differentiated from European ash tree (t25).
European ash is a broad, spreading, deciduous tree, capable of reaching 35 m in height. It is distinguished from other species of ash in that it has black and not brown buds. The bark is light grey (smooth in younger trees, rough and scaly in older specimens). The dark green leaves are 20-35 cm long, pinnate compound, with 9-13 leaflets. The leaflets have sharply toothed margins. The leaves of the European ash are often among the last to open in spring, and the first to fall in autumn in the event of an early frost. They usually drop off while still green, but on some cultivars may turn yellow first.
European ash is dioecious (male and female flowers are distinct, and – most of the time, in this case – grow on separate trees). Unusually, a tree that is male one year can produce female flowers the next, and a female tree can become male. The non-ornamental, purple or greenish-white, petal-free springtime flowers (the female ones somewhat longer than the male ones) open before the leaves. Ash sheds copious pollen. The tree is entirely wind-pollinated. The pollen is carried as far as 110 m from the point of dispersion. Ash pollen load may vary extensively between years. (1) In Europe, European ash flowers in April and May.
The fruits are 4-5 cm long, oblong, and winged (known as ‘Ash keys’ because they hang in bunches). They turn brown and remain on the trees until the following spring, when they are blown off and carried away by the wind.
European ash tree pollen may often be overlooked as a cause of pollinosis, as the flowering season coincides with that of birch. Because of the close family relationship with the olive tree, the pollen is a significant cause of respiratory allergy through cross-reactivity. Observations from Switzerland suggest that European ash tree pollen may be, at least locally, as important as birch in the elicitation of spring pollinosis. (2)
European ash grows best in deep, well-drained, moist soils with other hardwoods. It is often utilised as a landscape tree and is commonly found in gardens and parks.
Because of its high resistance to splitting and its flexibility, European ash wood is the traditional material for tool handles, tennis racquets and snooker cues.
The bark contains the bitter glucoside fraxin, the bitter substance fraxetin, and tannin, quercetin, mannite, and malic acid.
Extract of Fraxinus excelsior combined with other ingredients, e.g. extracts of trembling poplar tree (Populus tremula) and goldenrod (Solidago virgaurea), has been used for the management of mild to moderate rheumatic complaints. (3)
European ash tree pollen contains Fra e 1, a major allergen related to the major olive allergen Ole e 1; the panallergen profilin Fra e 2; a calcium-binding protein, Fra e 3; a pectinesterase-like molecule; and an allergen sharing epitopes with Group 4 grass pollen allergens. (4, 5)
The following allergens have been characterised:
Fra e 1. (1, 4, 6, 7, 8, 9, 10, 11)
Fra e 2, a profilin. (1, 4, 10, 11)
Fra e 3, a calcium-binding protein. (1, 4, 10, 11)
Fra e 9, a 1,3-beta-glucanase. (12)
Fra e 12, an isoflavone reductase (IFR). (13)
Fra e 1 is a major allergen for ash pollen-sensitised individuals in northern and central Europe. It belongs to the olive tree Ole e 1-like family, and has a high degree of cross-reactivity with other members. (6) Approximately 70-80% of ash pollen-sensitised individuals will have Fra e 1-specific IgE antibodies. (1, 7)
Approximately 75% of European ash-sensitised patients were shown to have IgE antibodies to rFra e 1, and 29 of 30 ash-sensitised patients were shown to have positive SPT to rFra e 1. (7)
About 50% of ash-allergic individuals have been shown to be sensitised to Fra e 2, a profilin, and in the same study, the calcium-binding protein Fra e 3 was shown to be a major allergen. (4)
In a study evaluating the frequency of IgE-binding to Fra e 1 and pollen panallergens, Fra e 1 sensitisation was found in 100% of monosensitised patients (n = 6), 93% of oligosensitised patients (n = 16), but only 44% of polysensitised patients (n=25). IgE antibodies against Fra e 2, Fra e 3, and carbohydrate epitopes in the 3 groups were found in 0/0/17%, 0/19/31%, and 32/72/60% of the patients, respectively. The study found that only 20% of positive SPT to European ash resulted from cross-sensitisation to pollen panallergens. (1)
A recent study examined the repertoire of ash pollen allergens and the extent of the diversity of the IgE response in 114 ash-allergic patients. IgE- and non-IgE-reactive proteins were identified. Eighty-six per cent of sera showed binding to Fra e 1, 23% to Fra e 2, 3% to Fra e 3 and 57% to High Molecular Weight allergens (>30 kDa). Individual analysis showed several IgE-binding protein areas, out of which three were most often recognized: (i) Fra e 1 comprising at least 15 isoforms; (ii) a series of acidic spots (45 kDa); and (iii) Fra e 2, the ash profilin. HMW allergens comprised two unidentified proteins, one homologous to beta-galactosidase and the other to sugar transport proteins. A malate dehydrogenase and calmodulin were shown to be IgE-binding proteins, and 10 non-IgE-reactive proteins were identified. (11)
The genus Fraxinus belongs to the family Oleaceae. It is closely related to jasmine (Jasminum), lilac (Syringa), privet (Ligustrum), forsythia (Forsythia) and olive (Olea). Extensive cross-reactivity between the different individual species of the genus could be expected, as well as (to a certain degree) to members of the family Oleaceae. (14) Indeed, cross-reactivity has been documented among olive (Olea europaea), white ash (Fraxinus americana), privet (Ligustrum vulgare), and Russian olive tree (Elaeagnus angustifolia) pollen allergens. (15) A high degree of cross-reactivity has also been demonstrated among olive tree (Olea europaea), European ash (Fraxinus exselsior), privet (Ligustrum vulgare) and Phillyrea angustifolia, although there was no 100% identity among these species. (16) Similar results have been reported by other studies. (1, 17, 18, 19) Six common protein bands were found to be responsible for the cross-reactivity, with apparent molecular weights of 49.6, 40, 36.7, 19.7, 16.7, and 14 kDa. (19) A study using recombinant Fra e 1 reported an 82%, 88%, and 91% identity with, respectively, Syr v 1 (lilac tree), Ole e 1 (olive tree), and Lig v 1 (privet tree). (7)
Further clarity on the relationship between Oleaceae pollen allergens was achieved in a study evaluating the common epitope determinants in olive and other Oleaceae pollens: European ash, privet, lilac, and forsythia; 18- and 20-kDa proteins were present in each pollen, except for Forsythia. IgE antibodies to forsythia were directed at 50- to 55-kDa protein bands. (20)
A study comparing the profiles of olive and ash pollen allergens investigated degrees of cross-reactivity, using Spanish and Austrian allergic patients selectively exposed to olive or ash pollen. Both groups exhibited an almost identical IgE-binding profile to both pollen allergens, with major reactivity directed against Ole e 1 and its homologous ash counterpart, Fra e 1. Extensive cross-reactivity was demonstrated between olive and Ash pollen allergens. However, whereas cross-reactions between profilins and calcium-binding allergens also occur between unrelated plant species, cross-reactivity to Ole e 1 was confined to plants belonging to the Oleaceae. The study concluded that Ole e 1 is a marker allergen for the diagnosis of olive and ash pollen allergy. (8)
As Fra e 1 has strong cross-reactivity with Ole e 1, (6) some authors have suggested that the cross-reactivity is so pronounced that immunotherapy with Ole e 1 would protect European ash-allergic individuals. (19)
Since European ash pollen contains a profilin and a calcium-binding protein, and both are panallergens, cross-reactivity with other plants containing these protein families may result. In a study evaluating sera of 40 ash pollen-allergic individuals, 30% had IgE antibodies to several high-molecular-weight ash pollen allergens, cross-reactive with Timothy grass and olive pollen. (4)
Ole e 9, an olive tree pollen allergen and a 1,3-beta-glucanase, has been shown to have counterparts in pollen from European ash and birch tree pollen, tomato, potato, bell pepper, banana and latex. (12) Specific cross-reactivity between ash and birch has been reported in a study. (21)
IgE mediated reactions
European ash is a common cause of asthma, allergic rhinitis and allergic conjunctivitis, (1, 15, 22) in particular in central and northern Europe, as has been recognised in recent years. (1, 4) The tree is regarded as contributing to important health problems. (16, 23) In patients living near Strasbourg, ash pollen induces nearly 4% of the total sensitisation of the allergic population. (7) The relevance of ash pollen as a cause of sensitisation may have been undervalued because of the overlapping of the tree’s pollination period with the pollination of other, better-documented trees, such as Birch. (24)
A Spanish study was carried out in the Basque region, with the aim of demonstrating the importance of European ash tree pollen as a triggering factor in the allergic symptoms shown in early spring. In an area where European ash trees are common but olive trees are not present, 48 pollen-allergic patients were selected and classified into 3 groups in accordance with their predominant sensitisation: Olive tree pollen-allergic patients (O), grass-allergic patients (G) and olive tree pollen- + grass-allergic patients (M). A hundred per cent of O patients, 40% of M patients and 16% of G patients reported early symptoms, coinciding with the flowering of ash, when grass pollen was not yet present. Conjunctival challenge tests with ash and olive pollen extracts were positive in 70% and 100% respectively in O patients, 50% and 78% in M patients, and 31% and 58% in G patients. Conjunctival challenge tests with olive tree pollen extract in patients who suffered from early symptoms were positive with lower concentrations of the extract. The patients with early symptoms had a higher rate of positive skin-prick tests with ash pollen, and had significantly higher levels of IgE antibodies to European ash pollen than did late-symptomatic patients. The study concluded that ash pollen can be considered a potential cause of hay fever in areas where it is present in considerable amounts. (25)
In an eastern Austrian study of 5 416 consecutive patients sensitised to various pollens, approximately 17.6% were shown to have skin reactivity to ash. (1) Ash tree pollen has also been demonstrated to be an important aeroallergen in Zurich, Switzerland, contributing greatly to the overall pollen count. (26)
The daily pollen concentration measured in the atmosphere over a 6-year period in Badajoz, in south-western Spain, demonstrated that pollen from Fraxinus species was one of the most important aeroallergens. (27) In Madrid, Spain, pollen from Fraxinus species was shown to be among the dominant pollens from January to April. (28) In Cordoba, Spain, SPT was carried out on 1 500 pollen-allergic patients with an extract of Fraxinus pollen, and 59% were positive. The great majority of the patients were polysensitive; only 8% were found to be monosensitive. (29)
In Plasencia, Spain, aerobiological studies reported that, although the most common pollens detected were Quercus, Poacea, Olea, Platanus, Pinus, Cupressaceae, Plantago, Alnus, Populus, and Castanea, in 210 patients with a diagnosis of pollinosis, 68% were sensitised to European ash tree pollen. The percentages of sensitisation were the following: Dactylis glomerata, 80%; Olea europea, 72%; Fraxinus excelsior, 68%; Plantago lanceolata, 63%; Chenopodium album, 61%; Robinia pseudoacacia, 49%; Artemisia vulgaris, 44%; Platanus acerifolia, 37%; Parietaria judaica, 36%; Populus nigra, 32%; Betula alba, 28%; Quercus ilex, 21%; Alnus glutinosa, 21%; Cupressus arizonica, 8%; and Castanea sativa, 7%. (30)
Pollen from this species was also shown to be important in Mexico City, (31) Sivrihisar, Turkey, (32) and Vinkovci in northeastern Croatia. (33)
In St. Louis, Missouri, USA, in a study using SPT for pollen from 12 wind-pollinated tree species, it was reported that the closely related white ash had sensitised some individuals, but was less reactive than other tree species. (34)
Although ash tree pollen may not be a major component of aeroallergens in a given community, individuals who are olive tree pollen-allergic may also be found to be sensitive to ash tree pollen as a result of cross-reactivity. In a study of 503 patients with allergic rhinitis in the southern part of Switzerland (Canton Ticino), patients were evaluated by SPT for sensitisation to common allergens. The authors suggested that out of the 54% who were positive to olive tree, a great many would be allergic to ash tree pollen. (35) Similarly, in a study of olive tree pollen allergy, almost all patients exhibited concomitant skin sensitivity to both ash and olive pollen. (19)
Rhinitis and occupational asthma from exposure to ash wood dust has been reported. (36) A study described an 18-year-old man who worked in a furniture factory and reported rhinitis and asthma after exposure to ash wood dust. A bronchial provocation test induced a dual asthmatic response. Intradermal testing with ash wood extract elicited an immediate positive response. IgE antibodies to ash wood were found in the patient's serum. (37)
Compiled by Dr Harris Steinman, email@example.com
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