Tree pollens

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Booklet: Tree Pollens
Allergy - Which allergens? Phadia AB, 2005.
 

Trees are an important cause of pollen allergy. One reason for this is that they are both widespread and dense in the human environment, probably accounting for most of the world's pollen plants under cultivation. From ancient times they have been considered beautiful and in some cases sacred, and they have had a number of vital uses in human culture, producing not only food but material for shelter, fuel and implements. Trees are so congenial to most people that they are accompaniments to buildings (especially homes) almost wherever space, climate and soil allow. The intensely developed metropolis, and the impoverished, environmentally degraded rural village or urban slum in the developing world, are considered unwholesome in part because of the absence of trees.

As far as allergy is concerned, deforestation and urbanisation in the world overall are tendencies that need to be considered in balance against reforestation and suburbanisation in many industrialised countries – which are the main territory of allergy and the main focus of allergy studies. Also, the increasing preference in Europe, America and the Far East is to provide for “green space” in urban planning (while the newer cities of Australia and New Zealand have tended to be heavily “green” from the beginning), and trees are normally included. The public health benefits of trees' absorption of carbon dioxide from human activities are held to far outweigh any detriments from tree pollen. Tree pollen allergen exposure therefore remains high in the developed world, or is even increasing. Exacerbating the situation for pollen-allergic individuals is the preference for male trees as ornamentals, in order to avoid the trouble of harvesting or cleaning up from the ground the fruit of female trees. When a species is not self-fertilising, it is of course the male tree that produces the pollen, and this must be light, mobile pollen – with far greater allergenic potential.

The second reason for the importance of trees in allergy is their huge production of pollens. This must, however, be taken in the context of individual tree species: the gymnosperms or cone-bearing trees (mostly evergreens) are the biggest pollen shedders, but within this group the Pine is relatively benign, because its large pollen tends to fall straight down and not travel – but Cypresses, Junipers and Cedars are among the most troublesome of all allergenic trees. But the angiosperms or flowering trees (mostly deciduous) are also important, particularly Birch, Alder, Beech, Elm and Hazel. Because of human mobility and intervention in the environment, moreover, the allergy diagnosis and management that are necessitated by exposure to such allergens have been complicated. Olive trees, often planted for their fruit, now occur in many places where the climate is suitable (including southern Africa), not just in the Mediterranean. Cedars and Birches may still be considered characteristic of Japan and Scandinavia respectively, but this allows no firm deductions as to where specimens of these trees cannot have been introduced, beyond the most obvious places (Antarctica, the Congo basin, the Sahara, and a few others). The Eucalyptus was exclusive to Australia before colonial settlement of that continent, but is now one of the most widespread “alien” trees, and in many places a significant pest. The worldwide mix of trees is probably more diverse now than at any other time in human history, with all the problems this implies for dealing with allergy.

Allergen exposure
As far as allergenic trees are concerned, the “pollen season” can begin in early spring with the flowering of trees such as Birch, long before grass and weed pollens. On the other hand, some trees flower in autumn or twice a year, which can confuse expectations about pollen exposure. Rhinitis, asthma and conjunctivitis are typical in tree pollen allergy, and as with grass and weed pollen allergies, there is a tendency to accept discomfort and not pursue suitable diagnostic and management strategies, opting either for no treatment or possibly inappropriate chronic medication. But the influence of tree pollens on asthma, and  important patterns of cross-reactivity, make it very desirable that tree pollen allergy be adequately addressed clinically.

Cross-reactivity
Cross-reactivity can be expected to roughly follow botanical relationships. The classification in figure 1 on the next page (representing widespread if not universal taxonomical opinion) shows only those allergy-connected trees that are most closely related, and proceeds vertically from the largest groupings to the smallest.

Trees can be divided into two groups. The first is represented by the order Fagales, containing the families Betulaceae, Corylaceae and Fagaceae. These families are closely related and cross-reactivity can occur between members of different families within the order. The second tree group contains a variety of non-related families where crossreactivity can be expected only between members of the same family (1-13). Broader cross-reactivity encompasses weed and grass pollens and plant-derived foods and other substances from distantly related species. Oral Allergy Syndrome, especially relating to Birch pollen, is a phenomenon that is of particular interest (14). To sketch a general pattern, Olive tree and Birch are most prominent in studies of crossreactivity with, most strikingly, Mugwort, Wall pellitory, Plantain ragweed, Timothy, Rye, and Cocksfoot among the weeds and grasses; among foods and other substances, Apple, Celery, Melon, Carrot, Kiwi and Latex are most clearly significant in the same connection (15-24). It should be noted that the above is only a sampling of crossreactivity among and beyond tree pollens.


 

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