How hayfever works: The Full Evidence

Which tree pollen species cause the most allergic rhinitis in the UK and when is their season?

What the research says

The major tree pollen species causing allergic rhinitis in the UK are birch (Betula spp.), alder (Alnus spp.), hazel (Corylus spp.), oak (Quercus spp.), ash (Fraxinus spp.), and plane (Platanus spp.), with the tree pollen season running broadly from late February through to early June. Birch is the dominant tree allergen in Northern and Central Europe and is significant in the UK, though less so than in Scandinavia; early-season trees (alder, hazel) begin in late February to April, birch peaks mid-March to early May, and oak, ash, and plane extend the season into June. Tree pollen collectively accounts for approximately 25% of UK hay fever cases, with grass pollen dominating overall prevalence at around 90%.

How it works

Tree pollen proteins, particularly Bet v 1 and its homologues across the Betulaceae/Fagaceae family (alder, hazel, oak, hornbeam), trigger IgE-mediated type 1 hypersensitivity reactions upon inhalation, causing mast cell degranulation and release of histamine and other mediators that produce rhinitis symptoms. Extensive cross-reactivity between these homologous allergens means sensitisation to one species frequently confers reactivity to others within the group.

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How does birch pollen allergy differ from grass pollen allergy in symptoms and severity?

What the research says

The available evidence suggests birch pollen allergy is more strongly associated with oral allergy syndrome (OAS) and pollen-food cross-reactivity (via Bet v 1 and Bet v 2 allergens) compared to grass pollen allergy, while both conditions produce classic rhinoconjunctivitis symptoms with severity correlating with ambient pollen concentrations. One database study (ALYATEC, n>2000) reported a slightly higher asthma frequency in birch-allergic patients (~30%) versus grass-allergic patients (~20%), though direct head-to-head symptom severity comparisons are lacking in the retrieved literature. Symptom recall is further complicated by overlapping seasons, as grass pollen symptoms can interfere with retrospective reporting of birch pollen symptoms.

How it works

Birch pollen's major allergen Bet v 1 shares structural homology with PR-10 proteins in fruits, nuts, and vegetables, driving food cross-reactivity via IgE cross-recognition, whereas grass pollen allergens (Phl p 1, Phl p 2, Phl p 5) lack this widespread food homology, resulting in predominantly aeroallergen-driven rhinoconjunctivitis without the same degree of food-related reactions.

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Which weed pollens (nettle, dock, mugwort, ragweed) cause significant UK hayfever?

What the research says

Weed pollens cause clinically significant hay fever in the UK, affecting approximately 20% of hay fever sufferers, though they are secondary to grass pollen which affects ~95% of pollen-allergic individuals. Mugwort and ragweed are explicitly recognised as UK allergenic weed pollens, with ragweed representing an emerging threat projected to increase four-fold in pollen concentration by 2050 due to climate change. Specific UK sensitization prevalence data for individual species (nettle, dock, mugwort, ragweed) remains limited in the published literature.

How it works

Weed pollens contain allergenic proteins including pectate lyases, defensin-like proteins, non-specific lipid transfer proteins, and Ole e 1-like proteins that trigger IgE-mediated sensitization and subsequent mast cell degranulation upon re-exposure. Nettle pollen co-occurs at high concentrations during the mugwort season (constituting up to 78% of late-summer pollen load versus 7% for mugwort), complicating attribution of symptoms to individual species.

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How common is multi-pollen sensitisation and does it worsen overall season length?

What the research says

Polysensitization is a frequent phenomenon in allergic rhinitis patients, with evidence suggesting it is more common than monosensitization, particularly in adults; the POLISMAIL study noted that monosensitized adult patients are rare, and multiple cross-sectional studies confirm polysensitization as the predominant pattern across diverse populations. However, direct quantitative evidence that multi-pollen sensitization specifically extends or worsens overall allergic season length is largely absent from the current literature. The available studies characterize sensitization patterns and associated clinical severity differences, but do not isolate season duration as an outcome measure.

How it works

Multi-pollen sensitization can arise through genuine co-sensitization to discrete pollen species or via panallergens (e.g., profilins, polcalcins) that share high structural homology across divergent plant families, causing broad cross-reactive IgE responses. Theoretically, reactivity to pollens from sequential flowering species—such as tree pollens in spring followed by grasses in early summer and weeds in late summer/autumn—would cumulatively span a longer symptomatic period than monosensitization to a single pollen type.

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Does the UK pollen calendar differ meaningfully by region (south vs north, urban vs rural)?

What the research says

UK pollen calendars differ meaningfully by region, with southern and central England experiencing earlier onset, longer duration, and greater intensity across major pollen types (grass, birch, oak, ash) compared to northern England and Scotland, where seasons begin approximately 1–2 weeks later and produce fewer high-count days. Urban areas generally show lower overall pollen concentrations and shorter seasons than rural counterparts due to reduced vegetation cover, though cities like London have unique urban-specific taxa (e.g., plane tree pollen) absent in rural calendars. Regional pollen calendars produced from UK Aerobiology Network monitoring data (2004–2013) formally quantify these spatial differences, replacing a single national calendar with location-specific guidance.

How it works

Latitudinal temperature gradients drive earlier phenological triggering of flowering in southern regions, where accumulated heat units (degree-days above threshold temperatures) are reached sooner in spring. Urbanisation alters local vegetation composition and microclimate, reducing dispersal-source density for most taxa while introducing ornamental species such as plane trees.