Recombinant allergens have made it possible to dissect the mechanisms of allergen-antibody interactions at a molecular level. It becomes clear that monoclonal human IgG antibodies as well as animal derived antibodies can block the interaction of specific IgE antibodies as well as the allergen induced allergic effector reaction. Using PCR technology and combinatorial plasmid vectors, recombinant antibody fragments can be produced and it has even become possible to isolate allergen-specific IgE Fabs out of combinatorial IgE libraries constructed from allergic patients lymphocytes. Recombinant Fabs will represent useful tools to study the IgE-allergen interaction as well as for the standardization of allergen extracts and quantitative allergen measurements. Moreover, allergen-specific recombinant Fabs which block the allergen-IgE interaction have to be considered as tools for local therapy in effector organs of allergic patients.