Antibody A or a fusion protein thereof specifically binding to a leptin receptor (leptin-R) or a leptin-binding protein (leptin-BP), as well as compositions and methods for the use of these antibodies or fusion proteins for quantitative analysis, for therapeutic purposes and for the preparation of therapeutic drugs. Also disclosed is a method for quantitative determination of leptin in a sample of solubilized or suspended leptin-binding proteins by using specific antibodies or fusion proteins according to the invention, as well as diagnostic agents and (diagnostic) kits containing this antibody or fusion protein.