Detailed understanding of how Abs of the IgE isotype interact with allergen at the onset of an allergic reaction is of great importance for deciphering mechanisms involved in the development of disease and may aid in the design of hypoallergenic variants. In this study, we have used a set of human monoclonal IgE Abs derived from the repertoires of allergic individuals, specific for the major timothy grass pollen allergen Phl p 1, to gain detailed information on the interaction between Abs and allergen. These allergenspecific IgE are to varying degrees cross-reactive toward both different allergen isoforms and various group 1 allergens originating from other grass species. The usage of human monoclonal IgE, as an alternative to polyclonal preparations or mouse Abs, allowed us to locate several important IgE-binding epitopes on the C-terminal domain of Phl p 1, all clustered to an IgE-binding "hot spot." By introducing three mutations in the IgE-binding area of the C-terminal domain we were able to significantly reduce its reactivity with serum IgE. In conclusion, our study shows the great potential of using human monoclonal IgE as a tool for studies of the molecular interactions taking place during allergic responses. Furthermore, we present a novel IgE-hyporeactive fragment with the potential to be used as a safer hypoallergenic alternative in specific immunotherapy than the pollen extracts used today.