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|Route of Exposure||Airway (inhalation) and food (Ingestion)|
|Latin Name||Betula Verrucose|
|Other Names||European White Birch, Common Birch, Birch, Birch tree, European White Birch, B. pendula|
One of the most common tree type pollens found in Central and Northern Europe is the pollen from the birch tree of Betulaceae family. It is one most common tree species of Europe producing pollen allergen. It grows best in a temperate climate with light, fertile and acidic soil conditions. It belongs to Order Fagles and Family Betulaceae. Its pollen grain size is less than 24.39 µm with a spheroidal shape and three protruding pores. Birch pollens are one of the key causes of asthma, allergic rhinoconjunctivitis, and allergic rhinitis symptoms. Molecular and biochemical characterization of birch has reported 7 allergen components including a major allergen Bet v 1 and minor allergens Bet v 2, Bet v 3, Bet v 4, Bet v 6, Bet v 7, Bet v 8.which can bind with immunoglobulin E (IgE). Among these Bet v 1 is the key allergen of silver birch triggering specific IgE reaction in approximately 90% of sensitized individuals. Approximately 70% of Among birch pollen allergic patients up to 70% of them due to IgE cross-reactivity with food allergen sources like almond, apple, carrot, etc. experience hypersensitivity reactions. The birch pollen allergen shows cross-reactivity with birch homologous group (alder, oak hornbeam, chestnut, and beech) and plant driven allergens homologs to Bet v 1. Currently, allergen immunotherapy is the safest route to therapy.
Common Silver Birch is a deciduous, single-stemmed tree growing approximately 30 meters tall with an age of 100 to 150 years. The Birch tree’s bark is characterized as silvery white and smooth surface and may get blackened and alter to fissured rectangular bosses. The Birch tree leaves are featured as doubly serrated with alternate triangular or ovate shapes. Birch flowering season is during late spring. Typically at this time, the leaves start to appear on the tree (2).
Generally, it is found in clearings and heathlands. It is also grown in house gardens and parks. Massive quantities of pollen are shed by birch trees ahead of the unfolding of the leaves. A single catkin is able to generate 6 million grains (3). Birch grows well on well-drained, light and fertile soils with acidic conditions (2). The genus Betula species like a birch tree, inhabit the northern hemisphere’s boreal and temperate climate zones (4).
Birch species belong to Family Betulaceae and Order Fagles. The Betulaceae family consists of birch, alder, hazelnut and hornbeam tree. The homologs birch group contains five tree species which are European white birch (Betula verrucose), alder (Alnus glutinosa), hornbeam (Carpinus betulus), hazel (Corylus avellana), and oak (Quercus alba) (5).
|Taxonomic tree of common silver birch (6)|
|Species||B. Pendula (syn. Betula Verrucose).|
Figure 1: Common Silver Birch Taxonomy (1)
Birch pollen has a small grain size with grain’s maximum axis of less than 24.39 µm. Its grain is a monad of size 10-25µ, spheroidal shape, three protruding pores and circular polar view. A dry pollen grain has an irregular shape with sunken interpectoral areas. Among the small size grain Betula family, birch grains are described by maximum oncus width less than 10.19 µm and small differences between the axes (<2.92 µm when compared to other grains) (7, 8).
In Europe,Birch pollen is a prominent elicitor of allergies. An Austrian study showed 16.3% of immunoglobulin E (IgE) reactivity to Bet v 1 in the adolescent population (5, 9). A German study showed 14.1% were sensitized to Bet v 1 (5, 10). Other studies in Switzerland and Denmark on the general adult population showed 7.9% and 13.7%, prevalence of birch pollen sensitization, respectively (5, 11, 12). Another study in Belgium for a tree pollen mix of Birch, hazel and alder reported a sensitization prevalence of 13.2% (5, 13). A US-based study in allergic patients showed significant sensitization rates to birch tree pollen (32.9%), suggesting the clinical importance of these tree pollens as allergens (14).
Birch Pollen is the most dominantly found and common source of seasonal allergies in Central and Northern Europe. The sensitization to birch pollen allergies has been found to be prevalent in the range of 8 to 16% in European countries (5).
A birth cohort study showed 764 Swedish children (age <4 years) showed 7.1 (95% CI, 3.3‐15.3) odds ratio, in Bet v 1 sensitized children at age 4, of getting allergic rhinitis symptoms at age 8 or 16 years (5, 15). A Swedish study demonstrated exposure to birch pollen allergen had no impact on the lung function of children (16). Children with birch pollen–allergy showed symptoms after eating an apple, potato or carrot had significantly bigger Skin Prick Test (SPT) reactions than children with no food allergy history (5, 17).
Birch grows throughout most of Europe up to central Siberia. Birch reaches southern European regions such as Italy, Greece and Iberian Peninsula (2). It is also present in central-northern Asia such as from Caucasus to Siberia, till China and Japan.
Birch pollen particles get inhaled through the airway and thus induce a response against the allergen. The allergen is carried on a particle (like mold spore, fluid droplet, pollen grain, etc.). The allergen-carrying particle size (5 to 20 micron) determines the deposition site (upper or lower airways). The allergen discharged into the mucosal fluid, go through the airway epithelial barrier (18).
A cross-reactivity response usually occurs in individuals who are exposed to birch pollen grains after the ingestion of fruits or vegetables. One of the ways to considerably change the solubility of some proteins and change allergenicity is through food processing. The allergenicity may increase by the digestive system due to the release of small soluble allergenic fragments from poorly soluble conglomerate (18).
A Swiss study conducted on a dataset collected from 1969 to 2006 showed an increase in overall temperature leading to a 15-day early onset of birch pollen season. (5)
Birch Flowering Onset monitoring
One of the methods to detect birch flowering onset is by modeled fine spatial scale forecasting system to estimate pollen exposure. This system is created using land surface temperature (LST) data combined with in situ flowering data. (19)
Long distant transport (LDT) Pollen Monitoring
A study conducted in Poland on LDT episodes occurring in May with pollen sources mainly from Russia showed data on clinically significant extension to Poland’s birch pollen season. (20). Another study indicated the birch pollens from Russia, Sweden, Baltic States, Germany and Poland were transported to Finland. Therefore, detecting birch pollen in relatively birch‐free areas (5).
A study conducted for over a period of 15 years in Warsaw to analyze birch pollen season on bases of season characteristics, seasonal pollen index (SPI) and various pollen concentrations. The study showed the highest allergy threat by birch pollen with SPI value of 14249 and Max concentration of 2476 grains/ m3 (21).
Effect of pollution
Pollen allergy symptoms can be aggravated due to exposure to elevated air pollution. As irritants air pollutants can stimulate airway inflammation and disturbance in the epithelial barrier homeostasis. Thus, facilitating the allergen access to allergic immune response eliciting effector cells. The production of reactive oxygen species can be induced by Ozone, (NO2) and particulate matter PM10 which aggravates allergic inflammation. A study by Grundstrom and Dahl in 2017 determined a strong covariation between high NO2/PM10 levels and high birch pollen count (22).
The seasonal significant airborne birch allergens attached to small particles may stimulate allergic rhinitis (AR). A US-based study in allergic patients showed significant sensitization rates to birch tree pollen (32.9%), suggesting the clinical importance of studying these tree pollens (14, 23).
The global prevalence of ARC has been estimated between 10 and 25%, reaching 42% in the selected pediatric population. A study in Chile reported 14.9% of birch pollen sensitization in adults with ARC(24). Among OAS patients 72.0% were reported to have rhinoconjunctivitis (25).
Respirable birch pollen fine particles of <7.2 μm in diameter containing Bet v 1 can trigger asthmatic responses. A study in Spain on pollen‐allergic patients population stated a significantly high proportion of patients showed asthma symptoms in Betula pollen sensitized and non–Betula‐sensitized patients (42 vs 23%; P = 0.003). Another study in Brussels on aeroallergen seasons from year 2008‐2013, observed a 3.2% (95% CI, P < 0.05) increase in asthma‐related hospitalizations was noticed due to an increase in birch pollen count (5).
A study conducted in Denmark stated the probability of getting allergy responses in patients with isolated birch pollen allergy to birch pollen-related allergenic food was 25%. To other pollens probability of co-sensitization was found to be 50% (p5c) (26). In Northern Europe patients with apple, allergy is found to be birch pollen allergic too as Mal d 1 (apple) is inefficient in inducing IgE antibodies (5).
The pollen-food allergy syndrome also known as Oral Allergy Syndrome (OAS) is caused by flavors, nuts, raw fruit, and vegetables. Usually found in the adolescent population and more characteristics with seasonal AR patients. OAS cross-reactivity reaction usually occurs in birch pollen allergic individuals who are exposed to birch pollen during the season and ingest apple, almond, carrot, apricot, peanut, peach, plum, caraway, hazelnut, pear, soybean, celery, coriander, fennel, aniseed or parsley (27).
Due to the ingestion of cross‐reactive food allergens, birch pollen allergic patients after approximately 6 to 48 hours may show deteriorating AD symptoms. These delayed AD effects in response to birch pollen-related food allergens seem to be facilitated by T‐cell cross‐reactivity instead of IgE cross‐reactivity (5).
Allergenic tension-fatigue syndrome in children and fatigue in allergic adult
Allergic children many times suffer from disturbed behavior characteristics. This change in behavior is mainly constituted by the primary allergic disorder of the nervous systems. The main variations that can be noticed in allergic children are allergic to tension or allergic fatigue (28). Allergic reaction to tree (Birch) pollen may cause mental dullness and fatigue. Some may experience dizziness, shivering, or hyperactivity (29).
AIT is the only therapy that can possibly change the progression of the allergic disease by inhibiting AR patients from the development of asthma and possible new allergen sensitizations (30).
A birch SLIT tablet clinical trial showed a considerable improvement in the principal endpoint of a total combined symptom and medication score. These improvements were noticed during both the birch pollen and total tree pollen season comprising of birch, hazel and alder (5). Several clinical trials have exhibited the advantage of allergen immunotherapy (AIT) via both routes of administration i.e. subcutaneous (SCIT) and sublingual tablet (SLIT) for birch‐related ARC (5).
One of the main measures to be taken is to cut down the amounts of pollen in the respiratory system. During pollen season allergic patients are suggested to stay indoors or wear masks outdoors to keep away from pollen allergens. In worsening conditions, patients may be suggested to move to live in a non-pollen area (31).
One of the important aspects of pollen-related- food allergy management is the detection and avoidance of cross-reactive allergy related food products. For particular allergy management patients may be advised to avoid symptom eliciting raw and processed fruits and vegetables. The individual suffering from continuing symptoms may be advised to eliminate allergic food after detection from the diet (27). Another strategy is by decreasing allergenicity by more extensive fragmentation for example by the loss of allergenicity via the damaging impact of cooking on many allergens. For example, shredding of apples, by oxidative browning of apple polyphenols. These results are due to a tannin-like structure that causes denaturation of apple proteins (18).
To date, a total of 7 allergens for Betula verrucose have been identified and formally named. (http://www.allergen.org) (32).
Bet v 1 birch allergen is one of the major Fagales pollen allergens and belongs to pathogenesis-related protein class 10 (PR-10). PR-10 group consists of aeroallergens and common food allergens. Birch Pollen minor allergens are Bet v 2 (profilin) , Bet v 3 (Polcalcin-like protein), Bet v 4 (polcalcin) , Bet v 6 (isoflavone reductase) and Bet v 7 (cyclophilin) (14).
Allergens for Silver Birch (32)
|Allergen||Biochemical Name||Molecular Weight (kDa)||Allergenicity|
|Bet v 1||Pathogenesis-related protein, PR-10, Bet v 1 family member||17||Approximately 95% of patients allergic to birch pollen, Bet v 1 is accountable for IgE binding|
|Bet v 2||Profilin||15||Allergic individual's IgE antibodies attached to natural and recombinant Bet v 2.|
|Bet v 3||Polcalcin-like protein (4 EF-hand)||24||IgE bind to rBet v 3 on nitrocellulose filters.|
|Bet v 4||Polcalcin||7-8||A study showed recombinant Bet v 4 was identified by 20% of birch pollen-allergic patients.|
|Bet v 6||PhenylCoumaran benzylic ether reductase||35||
Its obsolete name is Bet v 5.
In 32% of the sera extracted from patients allergic to birch pollen, IgE binding to recombinant Bet v 5 occurred with a CAP class > 3 on immunoblots.
|Bet v 7||Cyclophilin||18||Approximately 20.8% of birch pollen-allergic patients identified Bet v 7 on immunoblots.|
|Bet v 8||Glutathione-S-transferase (GST)||27||A cohort of birch pollen-allergic patients showed GST sensitization rates of 13% (p4b) (14).|
A study reported a correlation of pre-season nasal levels of Bet v 1–specific IgE, free light chain (FLCs), and Interleukin (IL)-8. The in-season symptom severity was found positive in patients with seasonal AR. In the same study, the level of the IL-33 receptor in the nasal fluid was positively correlated with symptom severity (33).
The birch homologs alder, beech, hornbeam, hazel, chestnut and oak extracts include allergens that in vitro are cross‐reactive to Bet v 1. In a study conducted in Germany of the individuals participating among those 92% were sensitized to Bet v 1. Among those all (100%) were also co-sensitized to pollen extract of alder and hazel (5, 34).
Nearly 70% of birch pollen–allergic patients experience inflammatory reactions due to cross-reactivity between birch pollen allergens and food proteins. An Austrian and a Swedish study on birch pollen allergic patients reported IgE‐facilitated reactions to apple (80% and 47%), to peach (51% and 34%), to walnut (41% and 26%), and to carrot (35% and 23%) (5, 35, 36) Food allergens cross reactive with Bet v 1 are apple (Mal d 1), kiwi (Act d 8), peach (Pru p 1), pear (Pyr c 1), strawberry (Fra a 1), apricot (Pru ar 1), cherry (Pru av 1), hazelnut (Cor a 1), peanut (Ara h 8), soybean (Gly m 4), celery (Api g 1) and carrot (Dau c 1) (37, 38). A study showed among the birch pollen allergic patients, 20.8% experienced a food allergy. Among these patients, OAS triggers were found to be apple (68.0%), peach (56.0%), nuts (36.0%), kiwi (20.0%), plum (16.0%), cherry (16.0%), and persimmon (20.0%) (39).
Author: Turacoz Healthcare Solutions
Reviewer: Dr. Magnus Borres
Last reviewed: October 2020