**2. Taxonomy of allergenic plants**

Excellent reviews have been made as regard to the taxonomical classification of the allergenic plants (Yman1982; Takhtajan 1997; D'Amato et al 1998; Mothes et al 2004; Mohapatra et al 2004; Esch 2004; Radauer et al 2006). Moreover, several broad databases have compiled profuse and well-documented information linking the most relevant plant allergenic sources, the identified allergens and their taxonomical classification. They include Pharmacia (Pharmacia Diagnostics, 2001) and later Phadia/Thermo Fisher Scientific (http://www.phadia.com/en/Allergen-information/ImmunoCAP-Allergens/Allergen-compo nents-list/), the Allergome database of allergenic molecules (Mari et al 2009; http://www.allergome.org/index.php) and the official site for the systematic allergen nomenclature approved by the World Health Organization and International Union of Immunological Societies (WHO/IUIS) Allergen Nomenclature Sub-committee (http://www.allergen.org/index.php). Independently of the widespread presence of crossreactivity, most allergens are described in these works and databases as characterized in a single species (e.g. rBet v 2 Profilin, Birch= *Betula verrucosa*). Only a minority are referenced to taxonomical entities different to species, either to a combination of related species, cultigens or hybrids (e.g. *Musa acuminata* / *sapientum* / *paradisiaca*) or to a heterogeneous group of more than one (often numerous) species (e.g. *Eucalyptus* spp. Note that these abbreviations are not italicized or underlined, and can easily be confused with the abbreviations "ssp." or "subsp." referring to subspecies.). In several cases, allergens are referred to taxonomic ranks of higher entity than species (e.g. *Theaceae*). Only a few allergens are univocally attributed to infraspecific plant categories like varieties (e.g. *Brassica oleraceae var. italica*, *var. gemmifera*, *var. capitata*, *var. botrytis*).

As regards to pollen allergen analysis, two alternatives, apparently opposite, although somehow complementary strategies are defined:

Mothes et al (2004) analyzed cross-reactivities to pollens of trees of the Fagales order, fruits and vegetables, between pollens of the Scrophulariales and pollens of the Coniferales. They proposed a classification of tree pollen and related allergies based on major allergen molecules instead of botanical relationships among the allergenic sources, suggesting Bet v 1 as a marker for Fagales pollen and related plant food allergies, Ole e 1 as a possible marker for Scrophulariales pollen allergy and Cry j 1 and Cry j 2 as potential markers for allergy to Coniferales pollens. Another work analyzed pollen allergen sequences with respect to protein family membership, taxonomic distribution of protein families, and interspecies variability (Radauer and Breteneder 2006). These authors managed to classify all pollen allergens known to date into a limited number of protein families, and divide them into ubiquitous (e.g. profilins), present in certain families (e.g. pectate lyases), or limited to a single taxon (e.g. thaumatin-like proteins). This approach provides invaluable help in issues like the prediction of cross-reactivity, the design of diagnostic methods and the assessment of the allergenic potential of novel molecules. A similar approach is described by Moreno-Aguilar (2008).

On the other hand, different authors are contributing to define the specific allergenic composition of pollens, going deeper into the taxonomical classification usually observed (this is, characterizing the allergenic composition of pollens at infraspecific level), and abounding into the analysis of pollen allergenic polymorphism. Advantages of such strategy have been outlined before (Alché et al 2007). Diverse examples of this strategy are depicted next.
