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Background The pollens from Parietaria Judaica and Parietaria officinalis are a major cause of pollinosis in Europe. Par o I (13.5 kDa) and Par j I (12 kDa), the major allergens from these species, are highly crossreactive. Methods We have immunoscreened a P. judaica pollen cDNA expression library with a rabbit antiserum specific for Par j I and with a serum pool from allergic patients. An immunopositive clone containing a 26 bp insert was further characterized. The insert sequence was determined and the beta-galactosidase fusion protein was partially purified by electroelution from sodium dodecyl sulfate - polyacrylamide gel electrophoresis (SDS-PAGE)gels. Results This fusion protein specifically and extensively inhibited Par o I and Par o I binding of a rabbit antiserum and of a serum pool obtained from allergic patients. The antifusion-protein antiserum obtained in a rabbit (anti 6a) specifically precipitated radioiodinated purified Par o I in the double antibody radioimmunoassay (DARIA) and competed with antibodies of sera from allergic patients for the binding to Parietaria pollen extract allergens by enzyme linked immunosorbent assay (ELISA). We investigated the prevalence of antibody response towards the 6a epitope in patients naturally sensitized to Parietaria. The presence of 6a specific IgE antibodies was assessed in the sera of 33 patients using inhibition assays. All sera had antibodies with this specificity: the extensive percentage of inhibition reached suggested that they dominated individual ab response. Conclusion In conclusion, the antibody response induced by natural exposure to the pollen of Parietaria appears to be higly focused on a single linear antigenic determinant of the major allergens which may play a relevant role in the development of clinical allergy. This report is, to our knowledge, the first description of a dominant linear epitope of a major allergen. [ABSTRACT FROM AUTHOR]