Pollen allergy is the most frequent indication for allergen-specific immunotherapy (AIT). Consequently, it is essential to find the right preparation for the patient. As is known from research, several allergens can be assigned to distinct homologous groups (birch group, grasses etc.). The reference allergens from each group can be used for diagnosis and treatment. Therefore, mixing homologous allergens (for example birch + alder + hazel) for AIT is no longer necessary. Furthermore, a mixture of non-homologous allergens (for example birch + grass + mugwort) can definitely no longer be recommended as this will lead to a reduction of allergen content for the single allergen and can cause interaction between the extracts. This can further lead to reduced success rates of AIT. Physicians should select one or two relevant allergens for their patient (adults, adolescents or children) for AIT and inject them in different solutions in one arm each. Pharmaceutical companies have to be requested to avoid the possibility to order non-homologous mixtures on their order forms and to consider the principle of homologous reference allergens in the selection of extracts.