A monovalent antibody of the IgG4 type, which comprises i) a variable region of a selected antigen specific antibody or an antigen binding part of the said region, and ii) a CH region of an immunoglobulin or a fragment thereof comprising the CH2 and CH3 regions, wherein the CH region or fragment thereof has been modified such that the region corresponding to the hinge region do not comprise any amino acid residues which are capable of forming disulfide bonds with an identical CH region or other covalent or stable non-covalent inter-heavy chain bonds with an identical CH region in the presence of polyclonal human IgG, and wherein and the constant region of the heavy chain has been modified so that one or more of the following amino acid substitutions have been made relative to the sequence set forth in SEQ ID NO: 4: Glu in position 225 of SEQ ID NO: 4, corresponding to Glu in position 357 according to EU index, has been replaced by Val, Ser in position 232 of SEQ ID NO: 4, corresponding to Ser in position 364 according to EU index, has been replaced by Arg, Leu in position 236 of SEQ ID NO: 4, corresponding to Leu in position 368 according to EU index, has been replaced by Ser or Thr, Asp in position 267 of SEQ ID NO: 4, corresponding to Asp in position 399 according to EU index, has been replaced by Ser, Phe in position 273 of SEQ ID NO: 4, corresponding to Phe in position 405 according to EU index, has been replaced by Arg, Gln, or Tyr, Tyr in position 275 of SEQ ID NO: 4, corresponding to Tyr in position 407 according to EU index, has been replaced by Gln, or Lys. i) a variable region of a selected antigen specific antibody or an antigen binding part of the said region, and ii) a CH region of an immunoglobulin or a fragment thereof comprising the CH2 and CH3 regions, wherein the CH region or fragment thereof has been modified such that the region corresponding to the hinge region do not comprise any amino acid residues which are capable of forming disulfide bonds with an identical CH region or other covalent or stable non-covalent inter-heavy chain bonds with an identical CH region in the presence of polyclonal human IgG, and wherein and the constant region of the heavy chain has been modified so that one or more of the following amino acid substitutions have been made relative to the sequence set forth in SEQ ID NO: 4: Glu in position 225 of SEQ ID NO: 4, corresponding to Glu in position 357 according to EU index, has been replaced by Val, Ser in position 232 of SEQ ID NO: 4, corresponding to Ser in position 364 according to EU index, has been replaced by Arg, Leu in position 236 of SEQ ID NO: 4, corresponding to Leu in position 368 according to EU index, has been replaced by Ser or Thr, Asp in position 267 of SEQ ID NO: 4, corresponding to Asp in position 399 according to EU index, has been replaced by Ser, Phe in position 273 of SEQ ID NO: 4, corresponding to Phe in position 405 according to EU index, has been replaced by Arg, Gln, or Tyr, Tyr in position 275 of SEQ ID NO: 4, corresponding to Tyr in position 407 according to EU index, has been replaced by Gln, or Lys. Glu in position 225 of SEQ ID NO: 4, corresponding to Glu in position 357 according to EU index, has been replaced by Val, Ser in position 232 of SEQ ID NO: 4, corresponding to Ser in position 364 according to EU index, has been replaced by Arg, Leu in position 236 of SEQ ID NO: 4, corresponding to Leu in position 368 according to EU index, has been replaced by Ser or Thr, Asp in position 267 of SEQ ID NO: 4, corresponding to Asp in position 399 according to EU index, has been replaced by Ser, Phe in position 273 of SEQ ID NO: 4, corresponding to Phe in position 405 according to EU index, has been replaced by Arg, Gln, or Tyr, Tyr in position 275 of SEQ ID NO: 4, corresponding to Tyr in position 407 according to EU index, has been replaced by Gln, or Lys.