A review. Recombinant DNA technol. provides the means for producing allergens that are equivalent to their natural counterparts and also genetically engineered variants with reduced IgE-binding activity. The proteins are produced as chem. defined mols. with consistent structural and immunol. properties. Several hundred allergens have been cloned and expressed as recombinant proteins, and these provide the means for making a very detailed diagnosis of a patient's sensitization profile. Clin. development programs are now in progress to assess the suitability of recombinant allergens for both s.c. and sublingual immunotherapy. Recombinant hypoallergenic variants, which are developed with the aim of increasing the doses that can be administered while at the same time reducing the risks for therapy-associated side effects, are also in clin. trials for s.c. immunotherapy. Grass and birch pollen preparations have been shown to be clin. effective, and studies with various other allergens are in progress. Personalized or patient-tailored immunotherapy is still a very distant prospect, but the first recombinant products based on single allergens or defined mixtures could reach the market within the next 5 years.