A Multicentre Randomised Placebo-controlled Double-blind Clinical Trial for Evaluation of Safety and Efficacy of Specific Immunotherapy With an Aluminium Hydroxide-adsorbed Allergoid Preparation of Birch Pollen Allergens

Type I allergy is an immune-disorder which stems from the formation of IgE antibodies against proteins and glycoproteins from plants, insects, animals and fungi, most of which for healthy subjects are considered to be harmless. However, in allergic patients the cross-linking of specific IgE-antibodies on effector cells by allergens activates an immunological cascade leading to the symptoms of Type I allergy including rhinitis, conjunctivitis, asthma, and anaphylactic shock. Pollens from wind-pollinated plants including trees, grasses and weeds, are amongst the most frequent and potent elicitors of Type I allergy. It is not possible to avoid exposure to these pollens and therefore the symptoms that patients inevitably suffer must be treated with either symptomatic medication or allergen specific immunotherapy.

The Betulaceae family includes the genera Alnus (alder), Betula (birch) and Corylus (hazel). Trees belonging to these genera are widespread in middle and northern Europe and, in combination with the fact that they shed large quantities of wind-borne pollen, leads to their allergenic significance. The prevalence of sensitisation to birch pollen has been studied, and in the case of a middle European (Viennese) population, for example, it has been demonstrated that approximately 40 % of patients with allergic rhinitis are sensitised. Although the pollen season for any one genera seldom lasts for more than a few weeks, the well-documented cross-reactivity between the different Betulaceae and other tree pollens of the Fagales order contributes to a protracted season of symptoms for many allergic patients.

Allergoids are prepared by chemical modification of partially purified native aqueous aller-gen extracts. Native allergen extracts are depleted of components with a molecular mass of less than 5000 Dalton by diafiltration prior to chemical modification with aldehydes. The modification causes a substantial reduction in the allergenicity of the extract as can be judged by skin prick testing, provocation testing, histamine release from sensitised leukocytes and measurement of IgE-binding activity by RAST-inhibition. However, immunogenic activity and T-cell reactivity are retained. These properties enable allergoids to be used as a basis for allergen specific immunotherapy with a reduced risk of inducing IgE-mediated side-reactions and the possibility of administering larger doses of immunogen over a shorter time course than with native allergens.