An allergy is a sensitivity to a normally harmless substance and some people possess an allergy to plant pollen, known as pollinosis, or more commonly ‘hay fever’ (ICD-10: J30.1). Allergenic pollens are those that cause pollinosis. Grass pollen is the major cause of pollinosis in many parts of the world (Freidhoff et al. 1986). Other sources of allergenic pollens include trees [birch is the major pollen-allergen-producing tree in northern Europe (Eriksson and Holmen 1996)], weeds [Parietaria is the main allergenic genus of the Urticaceae (nettle) family (D’Amato et al. 2007) and ragweed is the major allergenic pollen in North America] and ornamental plants [e.g. weeping fig, yucca, ivy, palm tree and geranium (Mahillon et al. 2006)].