Grass pollens are the principal causes of respiratory allergic disease globally. The IgE reactivity to these allergens is manifested by about 40% of allergic patients and 20% of the general population. Timothy grass (Phleum pratense) is one of the most common grass species of northern and central Europe that produces pollen allergens (1, 2). It grows best in cooler, humid, temperate climates, and is sown as a pasture (2, 3). It belongs to the phylum Spermatophyta, class Monocotyledonae, and subfamily Pooidaea (2). Timothy grass pollen grains are 22-122 micrometers in diameter with spheroidal to ovoidal and sometimes elliptical shape (4). It is one of the best characterized allergenic grasses since most of the commercially available components of grass pollen allergy originate from timothy grass (3). Allergenicity of timothy grass results from a number of allergens (1, 2). Molecular and biochemical characterization of P. pretense has revealed the presence of several allergen components, such as Phl p 1, 2, 4, 5, 6, 7, 11, 12, and 13 (5). Among these allergen molecules, Phl p 1 is the most important and the key allergen of timothy grass triggering specific IgE reaction in more than 90% of sensitized individuals, followed by Phl p 5 affecting 65-90% of grass pollen allergic patients (5, 6). Phl p 7 sensitization was not observed in pediatric population of age <5 years and was insignificant in adolescents, in contrast Phl p 12 sensitization was found to increase with age (5). The allergen molecules of timothy grass show extensive cross-reactivity with grasses belonging to the family Poaceae and subfamily Pooideae and some corn and maize species (7, 8).

Timothy grass pollen induces allergic rhinitis (AR), allergic rhinoconjunctivitis (ARC), and asthma in sensitized individuals (7). In industrialized countries, grass pollen-induced AR and allergic asthma is one of the main health problems (9). It has been found that AR usually precedes asthma and AR itself serves as a risk factor for asthma (10, 11). Grass pollen allergy diagnosis is based on a clinical history to identify associations between grass pollen exposure and symptoms, plus confirmation of the relevant allergens for each patient by in vitro testing. The in vitro testing involves assessment of total IgE level that helps to interpret specific IgE levels (6). Besides, in vivo tests, such as skin prick test using allergen extracts is most frequently used to diagnose an IgE-mediated allergic reaction (6) and nasal allergen challenge (NAC) test is used to diagnose AR caused by grass pollen allergens (12). Allergy testing, identification of external and internal allergens, and then reducing exposure to allergens are optimal in controlling and reducing the symptoms of AR and asthma (13). However, it is not possible for an individual to completely prevent the exposure to external aeroallergen, such as pollen and thus during peak pollen season many of the sensitized individuals are at an increased risk to develop allergic diseases (13).

Pharmacotherapy using nasal medications is the first-line treatment of respiratory allergic diseases such as AR and asthma; however, these medications have some limitations on patients’ quality of life (7). Allergic immunotherapy (AIT) with the ability to modify allergic disease, to prevent disease progression, and to provide long-term remission of allergy has been used as an alternative to pharmacotherapy nowadays (14).

Worldwide distribution

Timothy grass has a widespread distribution in the temperate climate. It is one of the best characterized allergenic grasses since most of the commercially available components for grass pollen allergy originate from timothy grass. Even though this grass pollen shows significant geographical distribution, it is the most common sensitizing pollen in Europe. Among the European population, it shows highest prevalence of sensitization, ranging from 18.5 to 28.5%. with a median prevalence of 16.9% (3, 6, 20, 21). In Austrian students, a prevalence of 36% was detected using allergen microarray analysis (6). Within the U.S., about 69% of the population is exposed to timothy grass (22). In Canada, the prevalence of sensitivity to timothy grass pollen usually varies between 20.2% and 23.3% with a median prevalence of 21.8% in patients with ARC (23). 

Risk Factors

Pollens were ascribed to be the risk factors of AR in the middle of the 19^th century (24). Grass pollens are known to be the most predominant seasonal aeroallergens that cause allergic symptoms such as ARC and asthma during summer in cool temperate climates. Many patients with ARC are allergic to timothy grass (7, 25). ARC and asthma are closely related and AR itself is considered to be a risk factor for  asthma (11, 25).

Pediatric issues

The average overall prevalence of ARC was 14.6% among older children of age 13-14 years. In the U.S., around 13% to 17% of children live with ARC (26, 27). According to the Public Health Agency of Canada report, more than 50% Canadian children suffered from AR and more than 20% from asthma (7, 28). ARC substantially affects the daily life activities of the school-going children causing disturbed sleep, limiting school, reduced school performance, and outdoor activities (26).

Worldwide distribution

Timothy grass is native to Europe, North Africa, and Northern Asia (also in parts of India and Pakistan). It has been introduced and widely cultivated as hay and pasture grass in North and South Africa, Australia, New Zealand, Greenland, Mexico, Canada, and the U.S., including Hawaii and Alaska, as well as Mauritius, Argentina, Chile, the West Indies, Uruguay, and Peru. Moreover, its cultivation has been introduced and started in Japan and in western, eastern, and northern China (17).

Main

Exposure to timothy grass pollens triggers ARC

Inhalation of grass pollens affects the upper respiratory system, leading to nasal irritation, sneezing, itching, rhinorrhea, and nasal blockage, which may be persistent or intermittent (29). 

Secondary

Entry of pollen allergens in lower respiratory tract to trigger asthma

Due to the size, the grass pollens (>10 µm) are stable in inhaled air getting deposited on eyes and nose and have lesser chance to enter the lower airways to stimulate asthma. However, grass pollen allergens once dispersed in the environment are found to have been linked with fine respirable particles (<10 µm) in the atmosphere. These micronic particles are breathable; they can enter the human airways and may trigger allergic reactions in the distal part of the lungs leading to asthma (30, 31).

Allergic rhinitis

AR is one of the most common allergic conditions in adults and a chronic condition in pediatric population. Grass pollen results in more than 50% of AR cases in adult and children and is the most predominant cause of respiratory allergies that greatly affects the quality of life of the individual (14, 32). In the U.S. and Western Europe, around 20% of the adult populations suffer from ARC (14, 33). Causal link between grass pollen allergens and AR have been detected in allergen challenge studies. Exposure to grass pollen allergen has found to have direct effect on the development of allergic respiratory diseases such as AR and asthma in the patients (34). The prevalence of confirmed AR among European adults varied between 17% to 28.5%. Recently, it has been observed that the prevalence of AR has increased in some countries with initial low prevalence (35).

AR occurs due to the inflammation of the inner nasal lining and is triggered when a person is exposed to specific allergen (grass pollen, dust mites etc.) and is mediated by IgE reactions. Nasal obstruction, rhinorrhea, nasal itching, post nasal drip, and sneezing are the symptoms of AR, which can be mild-to-moderate-to-severe (14).

Allergic rhinitis and allergic conjunctivitis occur together

When AR is associated with ocular symptoms, it is known as ARC. IgE-mediated ARC commonly accompanies AR and in developed countries its incidence is about 20% with a high comorbidity of AR. The common manifestations of ARC include itchy, watering or red eyes, and tearing (35, 36). In patients with even recognized AR, often ARC can go unreported (36).

Asthma

Co-occurrence of rhinitis and asthma (“United Airways Diseases”)

It has been found that AR usually precedes asthma and the AR patients with bronchial hyperresponsiveness (BHR) are more predisposed to asthma development (10). Moreover, it has been demonstrated that the subjects sensitized to grass pollens are at increased risk of developing asthma than non-sensitized person; however, this was not true for AR. In fact, AR itself has been found to be a risk factor for allergic asthma(11).In a study involving participants from different geographic areas in Europe, Oceania, and the U.S, the prevalence of asthma on pollen exposure was found to be 6 times more in subjects with AR than without AR (11). Around 20% of AR patients have been found to develop asthma eventually (10). According to ARIA guidelines (2019), early AR increases the risk of allergic asthma by 3.6 times in boys and 2.3 times in girls indicating  the importance of early causal treatment of AR (37). Moreover, it has been noticed that uncontrolled moderate-to-severe AR affects asthma control (35). These findings support the concept of ‘one airway, one disease’ that means asthma and AR may not be separate entities but rather interact with each other. Both AR and allergic asthma are the chronic inflammatory diseases of upper and lower airways  induced by allergens  (11, 38). Hence, AR and asthma can be unified by the concept of a “united airway disease” (38).

Grass pollen as a cause of allergic asthma

Investigators have associated risk of asthma to allergen sensitization and the asthma severity has been linked with the allergen exposure. In both adult and pediatric population, allergen sensitization is a risk factor for asthma. With the reduction of allergen exposure, asthma severity diminishes (39). The grass pollen related cytoplasmic fragments are anticipated to be the principal causative aeroallergen in Australian thunderstorm asthma epidemics (34, 40). In New Zealand, a group of children were followed up from birth till 13 years of age, the most common sensitization was found to be grass pollen, yet children sensitized to grass pollen did not show significantly increased risk for asthmatic development (39). In fact, conventionally, outdoor allergens are considered to the main risk factors of AR and indoor allergens for asthma (24). 

Atopic Dermatitis

There is a scarcity of studies showing the causal link between grass pollens and induction of atopic dermatitis (41).

In vitro diagnostics

Grass pollen allergy diagnosis is based on the clinical history to identify associations between grass pollen exposure and symptoms, plus confirmation of the relevant allergens for each patient by in vitro testing (6). The patients with suspected pollen allergy are subjected to the in vitro test, which involves assessment of total IgE level and measurement of specific IgE (sIgE) in serum (6).

Molecular assays of grass pollen allergy

 Allergen extracts are used for singleplex analyses, which involves a panel of purified natural and recombinant components commercially available for routine diagnosis. Moreover, allergen microarrays containing higher than 100 purified allergen molecules (multiplex analysis) help to measure IgE concurrently using only a small amount of blood. These multiplex assays aid in appropriate allergy diagnosis, tracking disease progression, and therapeutic outcomes (6).

Use of recombinant allergens to diagnose pollen allergy

 Molecular and biochemical characterization of timothy grass pollens revealed the presence of several allergen components. The production of the recombinant allergen molecules in Escherichia coli  (E. coli) and the utility of these recombinant molecules for molecular diagnosis of grass pollen allergy have been studied in different age groups (5). Recombinant timothy grass pollen allergens, which mimic the immunological properties of natural allergen molecules have been produced successfully. Individual recombinant timothy grass pollen allergens are found to be used consistently for in vivo and in vitro diagnosis of grass pollen allergy by successfully replacing conventional natural allergen extracts (42). 

Skin prick tests

Skin prick tests (SPT) represents one of the most commonly used in vivo diagnostic tools to confirm an immediate IgE-mediated allergic reaction in patients with ARC, asthma, food and drug allergy, AD, etc. (6). SPT can be performed using timothy grass pollen extracts. A study used recombinant hybrid molecules (HM) with 4 major allergens from timothy grass (Phl p 1, 2, 5, and 6) to diagnose grass pollen allergy by SPT. It detected that the recombinant HM resulted in accurate and specific in vivo diagnosis of grass pollen allergy in all tested patients, indicating a recombinant HM can replace traditional allergen extract for skin test-based diagnosis of grass pollen allergy (42).

Challenge tests

Nasal allergen challenge (NAC) test is an important tool to diagnose AR caused by grass pollen allergens. Allergen challenge test is a simple, safe, and straightforward technique, which helps to detect the specific allergens against which precise allergen immunotherapy can be initiated. In NAC, allergen is applied to the nasal mucosa to provoke an immediate type 1 immune reaction that represents the symptoms of AR. The clinical changes are then evaluated using either symptom scores or by measuring nasal potency (12).

Allergen immunotherapy

Pharmacotherapy is the most common therapy for AR/ARC and asthma. Nonetheless, many of the allergic patients reported that the nasal allergy medications result in lifestyle limitations (7). Allergen immunotherapy (AIT) modifies the fundamental immunologic mechanisms of allergic reaction and is thus considered as an essential alternative or complementary to pharmacotherapy (7). With the increase in number of patients with allergy, there is an increased use of allergen-specific immunotherapy with proven long-term benefits by preventing allergic progression (14).

Prevention strategies

Avoidance

Reducing exposure to grass pollens to fight ARC and asthma

Genetically susceptible individuals when exposed to increased allergens may develop allergic sensitization. Exposure to grass pollen allergens can enhance the risk of asthma and ARC or worsen symptoms in sensitized individuals in peak allergic season. Hence, identification of external and internal allergens followed by reduction of exposure to them may improve symptoms of AR and asthma (13). The optimal strategy to prevent the appearance of allergic reactions to timothy grass pollen and worsening of the bronchial symptoms is to reduce the exposure to these external allergens as much as possible (13).

Since, it is not possible to control the level of pollens in the outdoor environment, the main focus should be to control allergen in domestic setting. Indoor pollen exposure can be reduced by closing doors and windows during high pollen counts, using air conditioning with HEPA filters in home and car, bathing after coming from outside during pollen season to remove allergens from the body and head (13, 43).

It is recommended that the sensitized individuals should stay indoor in mid-day and afternoon in early and midsummer when pollen counts are highest. For adults, wearing glasses or sunglasses and face mask over the mouth and nose is advisable during mowing. If possible moving to other places where there is fewer pollen counts during peak pollen season is also recommended (43).

Effect of environmental conditions on timothy grass pollens

Exposure to outdoor pollen and environmental variation may trigger asthma or enhance the condition. Most of the pollen allergens are removed by the nasal mucosa and bronchial tract, only some submicronic pollen-derived bioaerosols (<5 um) can easily reach the lower respiratory tract. These bioaerosols are released by pollen ruptures when come in contact with water and they contain major allergens (43). During thunderstorm and rain, airborne pollen grains are washed and resulted in the formation of bioaerosol, which can trigger asthma exacerbation (34, 43). Increase in asthmatic attack following thunderstorm has been reported in Australia (34, 40). Therefore, for sensitized individuals, it is suggested to stay inside during thunderstorm in the pollen season (43).

Allergenic molecules

Allergenicity of timothy grass results from a number of allergens (2). The key to accurate management of timothy grass pollen allergy is to know the patient’s sensitization profile at molecular level. Molecular and biochemical characterization of P. pretense has revealed 28 allergen components, of which 15 can bind with IgE. Among them the major allergens are as follows (5, 44):

1. Phl p 1 (a major Group 1 allergen; Beta-expansin; 27 kDa),
2. Phl p 2 (10-12 kDa),
3. Phl p 4 (Berberine bridge enzyme; 55 kDa protein),
4. Phl p 5, (a major Group 5 allergen of 32kDa molecular weight),
5. Phl p 6 (11-12 kDa protein),
6. Phl p 7 (Procalcinin, a calcium-binding 6 kDa protein),
7. Phl p 12 (a 14.2 kDa profilin protein, previously known as Phl p 11),
8. Phl p 13 (Polygalacturonase, 55 kDa protein)

About 95% of grass pollen-allergic patients showed IgE binding to Phl p 1 and Phl p 5 allergens in pollen extract (44).

Cross-reactivity

An extensive cross-reactivity among different individual species of the genus Phleum is anticipated. Cross-reactivity is also there among the different members of the Poaceae family of grasses belonging to the subfamily Pooideae (18). Timothy grass is cross-reactive with the allergens present in other Northern pasture grasses, including perennial rye, lugrass/june, fescue, orchard/cocksfoot, sweet vernal, and redtop/bent/velvet (7) as well some corn, maize, and some monocot species (8, 18).

Other topics

Molecular Spreading

There is a clinical evolution of allergic disorders and increase in number of allergen sensitizations from  childhood to  adolescence. In children mono- or oligo-sensitizations are common whereas, in adolescents  polysensitization  is common (5). It has been reported that Phl p 1 is the major timothy grass pollen allergen that leads to IgE reactivity in more than 90% of allergic subjects, followed by Phl p 5 that causes IgE reactivity in about 65-85% of individual allergic to grass pollen of temperate region (1, 5, 6). Phl p 1 IgE has been detected ay all age groups, even in very young children, which is not true for Phl p 5 (5). Along with Phl p 1, this Phl p 5 is considered as a marker molecule for grass pollen allergy, and in some rare cases  where Phl p 1-specific IgE is absent, Phl p 5 gives dependable results of genuine sensitization (6). Phl p 7 sensitization was not observed in pediatric population of age < 5 years and was insignificant in adolescents  and frequency of sensitization of Phl p 12 was found to increase with the increase in age (from 30% to 36%); however this was not statistically significant (5). 

Author: Turacoz Healthcare Solutions

Reviewer: Dr. Christian Fischer

Last reviewed: October  2020