A polypeptide of the formula H-X1-Gln-Thr-Arg-Ala-Asn-Pro-Asn-Pro-Tyr-Thr-Ser-Arg-Arg-Ser-Val-Ala -Ser-X 2- Y in which X1 and X2 each represents an optional coupling-facilitating amino acid residue, and Y represents -OH or -NH2, and an artificial compound in free or carrier-associated form with the capability of binding to glyco-conjugates, especially immunoglobulins, which compound is chosen from the group consisting of said peptide and functional analogues and functional derivatives thereof, are disclosed. Additionally, there is described an artificial pertussis toxin antigen, which mainly consists of at least one peptide sequence reacting with antibodies induced by the native pertussis toxin selected from the above polypeptide and parts thereof. The above described artificial pertussis toxin antigen is included as a diagnostic antigen in a diagnostic immunoassay kit for the determination of antibodies induced by the native pertussis toxin in a sample of biological fluid, and as an immunizing component in a vaccine composition against whooping cough, respectively. Furthermore there is described an intradermal skin test composition comprising the above described artificial pertussis toxin antigen.
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