Introduction

Allergy is among the major diseases of the world affecting about 500 million people. These diseases are present in all countries, they pose a major burden to the society, and their prevalence is increasing in nearly all regions, with the fastest rate reported in developing countries. Even allergic diseases considered as non-severe, such as allergic rhinitis, impair the social and professional lives of the patients or their caregivers. Moreover, patients with allergic diseases have reduced learning capabilities and impaired school performance. Finally, the corresponding economic impact is substantial (Bousquet et al. 2006).

Allergic diseases result from a complex interaction between genes, allergens and co-factors, which vary between regions. Allergens are antigens reacting with specific IgE (immunoglobulin E) antibodies and mainly released in a form of molecular-weight fine aerosols from a wide range of mites, animals, insects, plants, or fungi. They are usually distinguished as indoor (mites, some molds, animal dander, insects) and outdoor allergens (pollens and some molds). Exposure to allergens is a trigger for symptoms in sensitised individuals with asthma and allergic rhino-conjunctivitis.

There are regional specifics of the sensitivity to allergens. For example, in Africa, allergic diseases are more common in urban rather than in rural areas possibly because parasites protect from atopic diseases. In most other regions the situation is the opposite suggesting non-allergenic co-factors in development of sensitization and symptoms.

Pollen is among the first identified and most important triggers of allergic asthma and rhino-conjunctivitis, and pollination depends on climatic and meteorological variables. Greater concentrations of carbon dioxide and higher temperatures may increase the amount of released pollen, change the geographical distribution of pollinating plants and induce longer pollen seasons. Pollen allergenicity can also increase due to a combined effect of pollen and air pollution (Laaidi 2001; WHO 2003).

According to the mode of the pollen transport, one can distinguish anemophil-ous and entomophilous plants.

The pollen grains of anemophilous plants are usually aerodynamic and disperse with wind. They represent a major problem for sensitised patients as they often are emitted in large quantities, may travel long distances and may affect individuals who are far from the pollen source. It is, however, those who are nearest the emission area or directly downwind from it who generally show the most severe symptoms. Severity, affected territory and duration of the pollinating season are mainly driven by actual weather conditions as well as by the larger-scale characteristics of the regional climate. Therefore, changes in environment and climate may in near future considerably affect loads of airborne allergens via changes in vegetation structure, magnitude of flowering and atmospheric transport. Understanding the mechanisms driving the development of pollen seasons and means of their short-and long-term forecasting for taking pre-emptive measures is of great importance to public health at present and in the predicted future conditions.

As shown in several studies (Erdtman 1937; Keynan et al. 1991; Rantio-Lehtimaki 1994; Campbell et al. 1999; Corden et al 2002; Latalowa et al 2002; Hjelmroos 1992; Damialis et al. 2005; Lorenzo et al. 2006), the pollen load in air consists of two main fractions: grains emitted from regional sources of pollen (nearby forests, grasslands, etc.) and grains released from remote sources and transported to the region with moving air masses. Forecasting the local and regional pollen emission can be based on various phenological models and observations, as well as on aerobiological monitoring. The long-range transport phenomenon poses different complicated problems, not considered until recently. Now, the long-range transport of both grains themselves and the allergenic material, which can be released during the atmospheric dispersion (Motta et al. 2006; Majd et al. 2004), is a quickly growing area of research. Large-scale pollen transport can also have environmental impacts well beyond the problem of human allergy. For instance, highlights the possibility of quick distribution of genetic material over large territories, across climate zones and vegetation regions. This can catalyze the changes caused by the altering climate and make them faster and more widespread.

The pollen of entomophilous plants is carried by insects. Attracted by the usually colourful and perfumed flowers they carry the pollen grains from one flower to the female parts of another. The pollen grains are sticky and adhere to one another, often forming heavy lumps, and to the antennae or other parts of the insects. Little pollen is liberated into the atmosphere and sensitisation thus usually requires direct contact between the subject and the pollen source.

Certain entomophilous plants, such as dandelions and spiraeas, produce large amounts of pollen which accidentally gets into the air. This is common when the flowers are pollinated by insects that consume a lot of pollen grains themselves, and when there is an open pollen presentation. Others are ambophilous, which means that they are adapted to wind as well as to insect dispersal, a system that allows for flexibility. Examples are some species of plantain, e.g. Plantago lanceolata, and manna ash, Fraxinus ornus. Some of these ambophilous species may be among the plants that cause allergy symptoms on a regular basis. The pollen causing most allergies are found among:

• Grasses that are universally distributed. The grasses pollinate at the end of spring and the beginning of summer, but, in some places such as Southern California or Florida, they are spread throughout the year. Bermuda grass (Cynodon dacty-lon) and Bahia grass (Paspalum notatum) do not usually cross-react with other grasses (Davies et al. 2005).

• Weeds such as the Compositeae plants: mugwort (Artemisia) and ragweed (Ambrosia) (D'Amato et al. 1998; Solomon 2001), Parietaria, not only in the

Mediterranean area (D'Amato et al. 1998), Chenopodium and Salsola in some desert areas (Al-Dowaisan et al. 2004). Weeds such as ragweed flower at the end of summer and the beginning of autumn. Parietaria often pollinates over a long period of time (March-November) and is considered as a perennial pollen. • And trees: birch (Betula), other Betulaceae (Lewis and Imber 1975; Eriksson et al. 1984), Oleaceae including the ash (Fraxinus) and olive tree (Olea europea) (D'Amato et al. 1998), the oak (Quercus), the plane tree (Platanus) (Varela et al. 1997) and Cupressaceae including the cypress tree (Cupressus) (Charpin et al. 2005), junipers (Juniperus) (Iacovacci et al. 1998), thuyas (Guerin et al. 1996), the Japanese cedar (Cryptomeria japonica) (Ganbo et al. 1995) and the mountain cedar (Juniperus ashei) (Ramirez 1984; Bucholtz et al. 1985).

Trees generally pollinate at the end of winter and the beginning of spring. However, the length, duration and intensity of the pollinating period often vary from 1 year to the next sometimes making the diagnosis difficult. Moreover, the change in temperature in Northern Europe has caused earlier birch pollen seasons (Emberlin et al. 2002). Multiple pollen seasons in polysensitized patients are important to consider.

The grass season is early to late summer, whilst weeds such as Ambrosia flower at the end of summer and beginning of autumn. Parietaria often pollinates over a long period of time (March-November) and is considered a perennial pollen. In warm and humid climate also grass pollens can be found all year round.

The size of the pollen varies from 10 to 100 |im on average. This explains why pollen itself is deposited in the nostrils and, more particularly, the eyes and also why most pollen-allergic patients suffer from rhino-conjunctivitis. However, pollen allergens can be borne on submicronic particles (Solomon et al. 1983; Suphioglu et al. 1992) and induce and/or contribute to the persistence of rhinitis and asthma. This is particularly the case of asthma attacks occurring during thunderstorms (Anto and Sunyer 1997; Bauman 1996; Bellomo et al. 1992; Knox 1993; Venables et al. 1997).

Cross reactivities between pollen types, due to the presence of homologous allergens with common epitopes, are now better understood using molecular biology techniques (Scheiner et al. 1997; Fedorov et al. 1997; Ipsen and Lowenstein 1997; Mothes et al. 2004). However, it is unclear as to whether all in vitro cross-reactivities observed between pollens are clinically relevant (Pham and Baldo 1995). Major cross reactivities include pollens of the Gramineae family (Freidhoff et al. 1986; Hiller et al. 1997; Mourad et al. 1988) except for Bermuda and Bahia grasses (Matthiesen et al. 1991; Lovborg et al. 1998) and Bahia grass (Phillips et al. 1989), the Oleacea family (Bousquet et al. 1985; Baldo et al. 1992; Batanero et al. 1996), the Betuleacea family (Hirschwehr et al. 1992; Mari et al. 2003) and the Cupressaceae family (Pham et al. 1994) but not those of the Urticaceae family (Corbi et al. 1985; Bousquet et al. 1986). Moreover, there is clinically little cross-reactivity between ragweed and other members of the Compositeae family (Leiferman et al. 1976; Fernandez et al. 1993; Hirschwehr et al. 1998). For the grass family, cross-reactivity is often extensive within subfamilies. Most of the common grasses within the temperate areas belong to Pooideae, and cross-react to a large extent. Bermuda grass (Cynodon dactylon) and Bahia grass (Paspalum notatum) belong to Chloridoideae.and cross-reactions between these species and Pooideae species are less common. Many of the reactions may in this way be explained by the close phylogenetic relationship between the pollen-producing plants. But in some cases, they may be the result of so called pan-allergens, which from a evolutionary point of view very conservative protein families. One example is profiling, present in all eukaryots, another is a family of pathogenesis-related proteins that occur in many angiosperms, and to whom the most important Betula allergen belongs. The occurrence of pan-allergens explain why distantly related plants such as banana (Musa) and melon (Cucurbita), or birch (Betula) and celeriac (Apium,) may be involved in cross-reactions.

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