Disclosed is an isolated antibody comprising a heavy chain (HC) immunoglobulin variable domain sequence and a light chain (LC) immunoglobulin variable domain sequence, wherein the heavy chain and light chain immunoglobulin variable domain sequences form an antigen binding site that binds to human FcRn and wherein the HC comprises: a HC CDR1 comprising an amino acid sequence at least 85% identical to EYAMG (SEQ ID NO:144) or VYAMG (SEQ ID NO:156), a HC CDR2 comprising an amino acid sequence at least 85% identical to SIGSSGGQTKYADSVKG (SEQ ID NO:145), or SIGSSGGPTKYADSVKG (SEQ ID NO:157), and a HC CDR3 comprising an amino acid sequence at least 85% identical to LSTGELY (SEQ ID NO:146), LSIRELV (SEQ ID NO:158), LSIVDSY (SEQ ID NO:164), LSLGDSY (SEQ ID NO:170), or LAIGDSY (SEQ ID NO:176)； and the LC comprises: a LC CDR1 comprising an amino acid sequence at least 85% identical to TGTGSDVGSYNLVS (SEQ ID NO:141), a LC CDR2 comprising an amino acid sequence at least 85% identical to GDSQRPS (SEQ ID NO:142), and a LC CDR3 comprising an amino acid sequence at least 85% identical to CSYAGSGIYV (SEQ ID NO:143).
