The present invention relates to peptide fragments which have one or more shared and/or similar amino acid sequences to amino acid sequences of specific portions of the 14 kDa protein of S. mansoni (SM14) or related FABPs (Fatty Acid Binding Proteins), the peptide fragments functioning as continuous or discontinuous epitopic regions of the molecule or mimicking its biological activity. More particularly, the present invention relates to a method for constructing active peptide fragments, peptide fragments, immunogenic compositions and diagnostic kits using peptide fragments.