Latin name: Alnus incana
Source material: Pollen
Common names: Grey alder, speckled alder
The Fagales order consists of trees from the Betulaceae family (grey alder and birch tree), along with hazel, hornbeam, oak, and chestnut tree.
Grey alder is the most widely distributed alder in Europe and western North America.
Grey alder is a rapidly growing, deciduous, multi-stemmed shrub or small tree, which tends to form thickets. It has a long trunk and a narrow crown. The alder typically grows between 2 and 5 m in height, but may reach 12 m. The bark is thin, smooth, and green-grey, greyish-brown, or reddish-brown. Trees often produce adventitious roots from near the base of the stem. The leaves are broadly elliptic or ovate, and dull green on both sides. The leaves remain green until they are dropped in the autumn.
Flowering generally begins during March or April, with seeds ripening from September to November. The flowers occur in catkins and are monoecious (individual flowers are either male or female, but both sexes are found on the same plant). Alders are wind-pollinated, and produce clouds of yellow pollen. The cones remain on the plants for about a year after the seeds are shed, aiding in identification during winter. The fruit is a small, single-seeded nutlet, with narrow lateral wings.
Grey alder seldom grows at a distance from water. It is typically found bordering streams, rivers and mountain springs, on moist lowlands and in swamps. It invades gaps and clearings in forests and thrives there.
More than 30 allergens have been detected in Alnus pollen, mainly from analysis of A. glutinosa. (1, 2, 3, 4, 5, 6)
No allergens have been characterised from grey alder tree (A. incana), but the following allergens have been characterised from A. glutinosa:
Aln g 1, a 17 kDa protein, a Bet v 1 homologue (Group 1 Fagales-related protein). (7, 8, 9, 10, 11, 12)
Aln g 2, a profilin. (13)
Aln g 4, a 9.4 kDa protein, a polcalcin, a calcium-binding protein. (12, 14, 15, 16)
Extensive cross-reactivity between the different individual species of the genus (e.g. between A.incana and A. glutinosa) could be expected. (17)
The Fagales order consists of trees from the Betulaceae family (grey alder and birch tree), along with hazel, hornbeam, oak, and chestnut tree. It is widely accepted that Fagales pollen allergies are initiated by sensitisation against Bet v 1, the birch pollen major allergen. A study investigated the allergenic potential of the (clinically, most important) Fagales pollen allergens from birch, alder, hazel, hornbeam, hop-hornbeam, oak, beech and chestnut; and showed that all the allergens showed the typical Bet v 1-like secondary structure elements, and were all able to bind serum IgE from Fagales-allergic donors. The data suggested that Bet v 1-like allergens of the Betuloideae and Coryloideae subfamily (subfamilies of the Betulaceae family) might have the potential to induce IgE antibodies with different specificities, while allergic reactions towards Fagaceae allergens are the result of IgE cross-reactivity. (18)
Studies of cross-reactivity have mostly concerned A. glutinosa, but because of the close relationship between this species and A. incana, their cross-reactivity is most probably similar. Close cross-reactivity has been demonstrated between birch, alder and beech pollen, (19) as well as between birch, alder and hazel (as a result of the common Bet v 1 homologous allergens Aln g 1, Bet v 1 and Cor a 1). (7, 4, 9, 20, 21) Cross-reactivity has also been demonstrated among birch, alder, hornbeam, hazel, European chestnut and oak. (13, 22, 23) Aln g 1 and Bet v 1 from birch tree have an 86.8% homology. (8) Cor a 1 isoforms from hazel pollen have been reported to have a 75.5-76.7% identity (83.6-85% similarity) with Aln g 1. (24)
Alder tree contains a calcium-binding protein, which may result in cross-reactivity with other pollens containing calcium-binding protein, e.g. Timothy grass, rye grass, birch tree, olive tree, mugwort, and ragweed. (25)
Patients sensitised to Japanese hop pollen have been reported to have a higher prevalence of skin-prick tests for (among other pollens) those of sunflower, Bermuda grass, orchard grass, alder tree, birch tree, and poplar tree. (26)
Cross-reactivity between birch tree pollen and the fruit of kiwi has been suggested. The amino acid sequence identity between the kiwi allergens Act d 8 and Act c 8 was shown to be 70%, and to Bet v 1 53% and 54% respectively, suggesting that that Bet v 1 homologues are allergens in kiwifruit, and of relevance for patients sensitised to tree pollen and kiwifruit. Act d 8 and Act c 8 were shown to have 50% and 51% homology with the Bet v 1 homologues in Aln g 1 from alder. (27)
IgE mediated reactions
Fagales pollen allergy represents the main cause of winter/spring pollinosis in the temperate climate zone of the Northern hemisphere. (18) Alder pollen (a member of the Fagales order) is a significant cause of asthma, allergic rhinitis and allergic conjunctivitis, in particular in springtime in middle and northern Europe, and in conjunction with birch and hazel pollen. (28, 29, 30, 31, 32, 33) The majority of studies have assessed either the Alnus species in general or A. glutinosa specifically; however, the close relationship between A. incana and other Alnus species, including A. glutinosa, suggests that a number of inferences can be drawn.
Allergy to alder is important in northern European countries and is increasing in southern Europe. Alder has been reported to be a significant cause of sensitisation or allergy in a wide range of geographic locations, including Genoa (a northern Mediterranean area in Italy), (33) southern Finland, (34) Fairbanks, Alaska, (35) Spain, (36, 37, 38) Norway, (39) Australia, (39) and Switzerland. (40, 41) In central Italy, the pollination period stretches from February to mid-October. Pollen count from alder is particularly high in the month of March. (42) Alder has also been reported to be relevant in Japan; (43, 44) however, Alnus sieboldiana Matsumura may be the predominant species. (45)
Studies have reported on alder pollen in Warsaw, Poland, (46) Sweden, (31) the Philippines, (47) and Tehran, Iran. (48) In Plasencia, Spain, Alnus was the eighth-most-prominent pollen found in an aerobiological study. Of 210 patients with a diagnosis of pollinosis, 20.9% were sensitised to Alnus glutinosa. (49) Alder pollen has been documented in aerobiological studies in Worcester, in the West Midlands, United Kingdom, (50) and in Rochester, Minnesota, USA. (51)
In an ‘All India Coordinated Project on Aeroallergens and Human Health’ study evaluating the prevalence of pollen allergens at 18 different centres in the country, the allergenically important pollens included Alnus. (52) However, in India a number of other Alnus species occur, including Alnus nitida, an important sensitiser in Delhi patients. (53)
As the amount of measurable pollen is highly dependent on geographic and climatic conditions, it varies considerably between different regions of a country, as documented in Switzerland, as well as between different countries and continents. (54) In the southern part of Switzerland, of 503 patients with allergic rhinitis tested by skin-prick for sensitisation to common allergens, 33% were sensitised to alder tree pollen. (29)
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, a statistically significant but small (<2%) effect was observed for Alnus. (55)
In Poznań, Poland, symptoms of patients with positive skin-prick tests to Alnus pollen allergens were the following: 51% pollinosis, 43% atopic dermatitis, 4% asthma, 1% chronic urticaria and 1% eczema. Serum-specific IgE for Alnus was class 5 or 6 in 21%. The authors considered that Alnus pollen is generally mildly allergenic, but that the amount of Alnus pollen released into the atmosphere in places such as Poznań may increase its impact on the population and make it one of the more important aeroallergens present. (56)
Alder pollen was reported to be a significant aeroallergen in Zagreb, Croatia. Patients with monosensitisation to birch pollen had the most severe symptoms in April. In the patients with poylsensitisation to alder, hazel and birch pollen who were cross-reactive, initial symptoms occurred as early as February, with abrupt exacerbation in March and April. The most severely affected patients were those allergic to birch, hazel, alder, grass and ragweed pollen, who had symptoms throughout the year, with exacerbation in spring and the late summer months. (57)
Compiled by Dr Harris Steinman, firstname.lastname@example.org
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