Leptin is primarily known as a hormone regulating food intake and the adjustment to a distinct state of nutrition. In the present work a new function of leptin will be described, namely the role of leptin in the immune system. Structural characteristics of leptin and the leptin receptor already suggest an immune modulatory function. The effect of leptin on the various populations of immune cells in vitro will be described. These data are supported by findings obtained in in vivo animal models as well as descriptive and functional data from leptin-deficient humans which will be outlined in detail. In conclusion, the chimeric function of leptin as hormone and a cytokine/adipokine represents a unique link between the immune and the endocrine system thus opening a new field of research and therapeutic options.