The present invention relates to conjugates comprising an antibody covalently bonded to a detectable label through a hydrazide thiol linker, wherein a hydrazide group of the linker is covalently bonded to an Fc region of the antibody, and wherein the detectable label is an enzyme having at least one thiol reactive group, wherein a thiol group of the hydrazide thiol linker is coupled to the at least one thiol reactive group of the enzyme. The present invention further relates to conjugates of an antibody and a modified phosphatase enzyme, the modified phosphatase enzyme comprising at least one thiol reactive group, wherein the antibody and the modified phosphatase enzyme are coupled through a mercaptobutyric acid hydrazide linker, and wherein a hydrazide group of the mercaptobutyric acid hydrazide linker is covalently bonded to an Fc region of the antibody.
