Provided are polymeric proteins that comprise two or more polypeptide monomer units, each monomer unit comprising two chimeric protein chains. Each chimeric polypeptide monomer unit comprises an Fc receptor binding portion comprising two immunoglobulin G heavy chain constant regions,wherein each immunoglobulin G heavy chain constant region comprises a cysteine residue which is linked via a disulfide bond to a cysteine residue of an immunoglobulin G heavy chain constant region of an adjacent polypeptide monomer unit. Each chimeric protein chain also comprises a modified immunoglobulin M tailpiece region, wherein the amino acid sequence of each chimeric polypeptide monomer comprises an alteration of the primary structure as compared to the native sequences from which the immunoglobulin G heavy chain constant region or immunoglobulin M tailpiece region are derived, and the alteration changes the number of glycosylation sites in a manner that promotes polymerisation. This promotion of polymerisation may lead to the generation of tetrameric, hexameric, and even dodecameric forms of the proteins. The proteins are suitable formedical uses,such as in the prevention or treatment of autoimmune diseases such as idiopathic thrombocytopenia. Also provided are methods of treatment using the polymeric proteins, and pharmaceutical compositions comprising the polymeric proteins.Linvention concerne des protéines polymères qui comprennent deux ou davantage de motifs monomères polypeptidiques, chaque motif monomère comprenant deux chaînes de protéines chimères. Chaque motif monomère polypeptidique chimère comprend une partie de liaison de récepteur Fc comprenant deux régions constantes de chaîne lourde dimmunoglobuline G, chaque région constante de chaîne lourde dimmunoglobuline G comprenant un résidu cystéine qui est lié par lintermédiaire dune liaison disulfure à un résidu cystéine dune région constante de chaîne lourde dimmunoglobuline G dun motif monomère polypeptidique adjacent. C
