The present invention relates to a polypeptide comprising 7 beta-strands A, B, C, D, E, F, and G sequentially connected together by connecting chains of amino acids, and a first alpha-helix sequentially located on the EF chain between beta-strands E and F, wherein the beta-strands are arranged so as to form a first beta-sheet comprising beta-strands A, B, D, and E, and a second beta-sheet comprising beta-strands C, F and G, said first and second beta-sheets being covalently bonded together so as to form a first Ig domain； wherein the EF chain between beta-strands E and F comprises the sequence X1-X2-X3-X4-K5H6 (SEQ ID NO:98), and X1, X3 and X4 are each independently any amino acid residue, characterized in that X2 is selected from the group consisting of A, G, I, V, L, R, S, T, Q, P, N, M, H, W, and pharmaceutically acceptable salts, stereoisomers, tautomers, solvates, and prodrugs thereof
