Understanding Pollen, Tree, and Flower Allergens: The Science Behind Seasonal Sensitivities

In the spring, millions of people around the world prepare themselves for the arrival of the pollen season. Tree and flower pollen are two primary culprits of seasonal allergic rhinitis, commonly known as hay fever. The biological mechanisms behind these allergens and their impact on human health are complex yet fascinating despite their familiar presence. This blog explores the science behind pollen allergens, their sources, and our insights from recent research.

What is pollen?

Pollen grains are microscopic structures produced by the male parts of seed plants, primarily trees, grasses, flowers, and weeds. In their biological role, they deliver male genetic material to female plant structures to enable fertilization. But! When pollen is dispersed into the air, it can become an airborne allergen for sensitive individuals.

Tree pollen is often lightweight and dry, ideal for wind dispersal (anemophily), which unfortunately facilitates inhalation by humans. Similarly, certain flowering plants produce airborne pollen that can also trigger allergic reactions, although many flowers rely on insect pollination and produce less airborne pollen.

According to the American Academy of Allergy, Asthma, and Immunology (AAAAI), tree and flower pollens are among the earliest seasonal allergens to appear, with many trees and spring-blooming flowers beginning to pollinate in late winter or early spring.

Major Allergenic Trees and Flowers.

The trees most commonly implicated in allergic reactions are:

• Birch (Betula spp.)
• Oak (Quercus spp.)
• Cedar (Juniperus spp.)
• Maple (Acer spp.)
• Elm (Ulmus spp.)
  

Examples of common flowering plants that contribute to pollen allergies are.

• Sunflowers (Helianthus annuus).
• Daisies (Bellis perennis).
• Ragweed (Ambrosia artemisiifolia) — though technically a weed, it has flowering structures that produce significant pollen.
• Chamomile matricaria chamomilla.
• Certain ornamental flowers such as lilies and chrysanthemums.

The flowers, such as sunflowers and daisies, produce substantial amounts of pollen that can become airborne, especially when cultivated.

Mechanisms of Allergic Response

Allergic reactions to tree pollen and flower pollen are driven by an exaggerated immune response:

1. When tree or flower pollen allergens enter an individual, their immune cells misidentify the harmless pollen particles as harmful, causing an immune response that results in the production of allergen-specific immunoglobulin IgE antibodies.
2. Upon subsequent exposures, these IgE antibodies bind to mast cells and basophils, specialized immune cells.
3. These interactions lead to cell degranulation, releasing histamines, prostaglandins, and leukotrienes—key chemical mediators responsible for symptoms like sneezing, nasal congestion, itchy eyes, and throat irritation.

Environmental and Climate Factors

Environmental changes, particularly climate change, have profound effects on the pollen season. Rises in global temperatures and increased atmospheric CO2 levels have been linked to longer pollen seasons and higher pollen counts, as demonstrated by a study published in Proceedings of the National Academy of Sciences. Since 1990, pollen seasons in North America have extended by an average of 20 days, with pollen concentrations increasing by 21 percent. Cities also increase pollen exposure. Urban heat islands and air pollution can increase both pollen production and its allergenicity. Ozone and other particulate matter can alter the protein structure of pollen, enhancing its ability to provoke immune responses.

Diagnosis

Accurate diagnosis of tree and flower pollen allergy typically involves a combination of patient history, skin prick testing, and specific IgE blood tests. Skin prick tests remain the gold standard due to their rapid results and high sensitivity.

Strategies to manage tree and flower pollen allergies

Strategies for management focus on both symptom control and allergen avoidance. Intranasal corticosteroids and leukotriene receptor antagonists are common pharmacological treatments. For long-term management, allergen immunotherapy (AIT) — either via subcutaneous injections (SCIT) or sublingual tablets (SLIT) — has shown effectiveness in reducing symptom severity and medication use over time.

Things you can do today:

• Monitor pollen forecasts locally.
• Keep windows closed during high pollen periods.
• Use HEPA filters in homes and vehicles.
• After outdoor activities, wash and change clothes.
• Limit outdoor exposure during peak pollen times (early morning).
• Consult an allergist to develop a personalized allergen-fighting plan.
  

For more information, read about the four pillars of allergen protection here.

Conclusion

As climate change and urbanization continue to alter pollen dynamics, ongoing research is crucial for developing effective strategies to manage and mitigate pollen-related allergies. Individuals affected by seasonally occurring allergies should work closely with healthcare providers to develop personalized evidence-based management plans.

References

• Larsen JN, Dreborg S. Standardization of Allergen Extracts. Methods Mol Biol. 2019;2020:63-76. doi: 10.1007/978-1-4939-9591-2_5
• L. Ziska, K. Knowlton, et al. Recent warming by latitude associated with increased length of ragweed pollen season in central North America, Proc. Natl. Acad. Sci. 2011, 108 (10) 4248-4251,
• W.R.L. Anderegg, J.T. Abatzoglou, et al. Anthropogenic climate change is worsening North American pollen seasons, Proc. Natl. Acad. Sci. 2021, U.S.A. 118 (7) e2013284118, https://doi.org/10.1073/pnas.2013284118
• Dhami S, Nurmatov U, Arasi S, et al. Allergen immunotherapy for allergic rhinoconjunctivitis: A systematic review and meta-analysis. Allergy. 2021; 72: 1597–1631. https://doi.org/10.1111/all.13201