Leptin is a protein produced by adipocytes that acts in the brain to regulate appetite and energy expenditure according to the amount of energy stored in tissue. There are several molecules involved in the control of metabolism which play an important function in the regulation of immune responses. Among these molecules, leptin has been shown to significantly influence both innate and adaptive immune responses and also in pathological conditions. Leptin is the product of the ob gene and is mainly secreted by adipocytes, although it has also been shown to be produced by T and B lymphocytes, natural killer cells and monocytes. It is a cytokine similar in structure to interleukin 2, an important T-cell growth factor. Further, recent studies indicated that leptin leads to stat3 activation in the hypothalamus through its receptors. However, leptin deficiency may lead to immune dysfunction leading to impaired cell mediated immunity (CMI). Higher and lower circulating levels of leptin may also lead to autoimmune disorders and infectious diseases. We discuss here the influence of leptin on the innate and adaptive immune responses and production of pro-inflammatory pathogenic cytokines. Thus, leptin is a mediator of the inflammatory response.