acquired by the peptide on complex formation with molecules of the immune system such as antibodies this approach is applicable to other protein antigens which present discontinuous epitopes according to the present invention the method for constructing peptides on the basis of the three dimensional structures of homologous molecules wherein said peptides are able to mimic or prevent the interaction between helminth pathogens and receptors said method consisting of i selecting regions of the parent protein which contain a high spatial density of residues which form continuous or discontinuous epitopes ii give priority to maintaining the previously predicted epitopic residues responsible for stimulating the desired immuno antigenic response within the selected peptide sequence iii elaborate sequences which whilst maintaining a high percentage of previously identified epitopic residues are of limited size varying from 8 to 28 residues iv two peptides may be chosen which may correspond to regions which are distant in the primary structure amino acid sequence but spatially close in the tertiary three dimensional structure the said regions are chosen on the basis that the three dimensional structure indicates that they can be readily united assuming that they retain their original structures v modifications may be made to the peptide be.