A highly allergenic fragment of the major timothy grass pollen allergen, Phl p 5, defined by a human monoclonal IgE antibody

We report the characterization of a domain of the major timothy grass pollen allergen, Phl p 5A, which contains the binding site for a human monoclonal IgE antibody. The human monoclonal IgE antibody fragment (Fab) was obtained from an IgE combinatorial library constructed from lymphocytes of a grass pollen-allergic patient. An 11.2-kD N-terminal fragment representing approximately one third of the complete Phl p 5A allergen could be identified to contain the binding site for the IgE Fab. The rPhl p 5A fragment revealed an extremely high allergenic activity in skin test experiments which in some cases equaled that of the complete Phl p 5A allergen. The rPhl p 5A domain thus represents an allergen fragment containing several IgE epitopes in a configuration optimal for efficient effector cell activation. We suggest the rPhl p 5A fragment and the corresponding IgE Fab as paradigmatic tools to explore the structural requirements for highly efficient effector cell activation.