The present invention relates to a method for the preparation of a conjugate comprising a first and a second polypeptide, said method comprising the steps of (a) incubating said first polypeptide in the presence of a heterobifunctional crosslinker comprising an N-hydroxylsuccinimide ester group and a maleimide group linked via a polyethylene oxide spacer; (b) removing excess heterobifunctional crosslinker; and (c) incubating the reaction product of step (b) with said second polypeptide, wherein said second polypeptide comprises at least one sulfhydryl group. Furthermore, the present invention relates to a conjugate obtainable by the method of the present invention. Also described is a pharmaceutical composition comprising the conjugate of the present invention and, optionally, a pharmaceutically acceptable carrier and/or diluent, and the use of the conjugate for the preparation of a pharmaceutical composition for preventing and/or treating an allergic disease or an autoimmune disease.