A polypeptide comprising a variant Fc domain, wherein said variant Fc domain has an amino acid sequence that differs from the amino acid sequence of a natural Fc domain because it does not comprise more than 15 amino acid residue modifications with respect to said Fc domain. natural, said amino acid residue modifications comprising a substitution of a lysine in residue 435 of Kabat and a substitution of aspartic acid in residue 288 of Kabat, wherein: i) said variant Fc domain is an Fc domain of human IgG1 variant and said natural Fc domain is an Fc domain of human IgG1； ii) said variant Fc domain is an Fc domain of variant human IgG2 and said natural Fc domain is an Fc domain of human IgG2； iii) said variant Fc domain is an Fc domain of variant human IgG3 and said natural Fc domain is an Fc domain of human IgG3； or iv) said variant Fc domain is an Fc domain of variant human IgG4 and said natural Fc domain is an Fc domain of human IgG4； and wherein said variant Fc domain shows: (A) enhanced binding affinity of said Fc domain of said polypeptide by human FcRn at least 2 times with respect to the binding affinity shown by a molecule comparable to human FcRn if it comprises said domain Natural fc； or (B) enhanced serum half-life of said polypeptide in humans of at least 1.5 times compared to the serum half-life of said polypeptide in humans if it comprises said natural Fc domain.Un polipéptido que comprende un dominio Fc variante, en el que dicho dominio Fc variante posee una secuencia de aminoácidos que difiere de la secuencia de aminoácidos de un dominio Fc natural porque no comprende más de 15 modificaciones de residuos de aminoácido con respecto a dicho dominio Fc natural, comprendiendo dichas modificaciones de residuos de aminoácido una sustitución de una lisina en el residuo 435 de Kabat y una sustitución de ácido aspártico en el residuo 288 de Kabat, en el que: i) dicho dominio Fc variante es un dominio Fc de IgG1 humana variante y dicho dominio Fc natural es un
