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The transfer of pollen from floral anther to recipient stigma is the critical reproductive event among higher plants – this is the botanical view of pollen. Proteins and glycoproteins from pollen can function as allergens, environmental molecules interacting with the human immune system to elicit an allergic response in susceptible individuals – this is how allergists and immunologists see pollen grains. Between 10 and 25% of the population now have symptoms of hay fever or allergic asthma and the incidence has more than doubled in the past three decades while the reason(s) for this increment are only hypothetical, but there is a multitude of them. Despite our natural focus on this impact of pollen on human health, pollen have to be considered in a larger context. First of all, to evaluate the bioavailability of allergens from pollen, we have to understand their function and their influence factors. Furthermore, pollen grains are not only releasing proteins eliciting specific immune responses, but they also liberate bioactive lipid mediators and this much more rapidly. And last but not least, recent observations indicate, that pollen do not only induce allergy and thus have a much broader impact on human health. This review is an attempt to favour this holistic view of pollen and their impact on human health.

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Background: The release of the aeroallergen Bet v 1 from pollen is a major determinant in the etiology of allergic airway disease due to birch pollen. Objective: We determined the release of the major birch pollen allergen Bet v 1 from pollen of birch trees growing in 2 different geographic regions in Germany for 2 consecutive years. Methods: Catkins were collected during pollination in 2002 and 2003 from 82 healthy trees in South (Munich) and West Germany (North Rhine-Westphalia). The release of Bet v 1 from pollen samples was determined by a Bet v 1-specific ELISA. Results: Pollen from South Germany released about 3 times more Bet v 1 than those from West Germany in both 2002 and 2003 (p = 0.034 and p = 0.007, respectively). This was independent of the number of pollen during the pollen flight season. In 2003, the release of Bet v 1 from pollen was more than 5 times higher than in 2002 in both regions (South Germany 6.1 times, p < 0.001; West Germany 5.4 times, p = 0.003). Conclusions: Despite large individual differences, there seem to be regional and year-to-year variations in Bet v 1 release from birch pollen. Therefore, the combination of pollen count and release of Bet v 1 from this pollen must be assessed to estimate Bet v 1 exposure reliably.

It is commonly believed that allergic sensitization starts when an allergen contacts the surface of an antigen-presenting cell in mucosal or skin epithelia. Most studies dealing with this aspect use allergen extracts as stimulus. Under natural exposure conditions, however, the bioavailability of allergen depends on allergen liberation from internal binding sites within the allergen carrier, e.g. pollen grains. In comparing total protein and major allergen release from timothy grass (Phleum pratense L.) pollen freshly collected on rural meadows or near high-traffic roads, there was a striking difference between the pollen, with higher allergen release rates from rural meadow pollen grains. Thus, allergen release does not explain the higher prevalence rates of atopic sensitization and disease observed in many epidemiological studies in children exposed to automobile exhaust. Therefore, other possible effectors from pollen grains were investigated. Pollen grains incubated in protein- free buffer were found to secrete significant amounts of eicosanoid-like substances, namely leukotriene (LT) B₄-like and prostaglandin E₂-like substances, in a pH-, time- and temperature-dependent fashion. The highest values of eicosanoid secretion were found in birch, grass and mugwort pollen, while pine (Pinus sylvestris L.) pollen showed only marginal eicosanoid-like secretion. Additionally, the release of these substances was significantly higher from pollen which had been collected near roads with heavy traffic, indicating a stronger proinflammatory activity of these pollen grains. In order to investigate the effects of air pollutants, native pollen grains were exposed in a dose- and time-dependent fashion in a fluidized bed reactor to traffic-related pollutants, e.g. volatile organic compounds (toluene, m-xylene), leading again to a significant increase in the secretion of LTB₄-like immunoreactivity, in contrast to exposure with sulfur dioxide. This finding opens a new dimension of understanding of the early events in allergic sensitization, indicating that proinflammatory effects of the allergen carrier, e.g. the pollen grain itself, can lead to activation of the mucosal membrane. These findings might help to also explain the higher prevalence rates of pollen allergy in areas with high automobile exhaust emissions. Furthermore, the allergenic ‘potency’ of various allergens has to be redefined at the allergen carrier level with regard to different stages of allergen and mediator release prior to the contact with the host’s immune system.