Leptin is an adipocyte-derived hormone and product of the obese (ob) gene. The role of leptin as an immunomodulator in inflammatory and immune responses has attracted increasing attention in recent years. Leptin plays an important regulatory role as an immunomodulatory factor in the maintenance of immune function and homeostasis. It mostly affects responses through the immunomodulation of monocytes, dendritic cells, neutrophils, NK cells, dendritic cells, T and B cell development and functions. Leptin is also an important inflammatory regulator, wherein higher levels influence the secretion rates of IL-6, C-reactive protein, and TNF-α. Meanwhile, leptin is highly involved in processes related to human metabolism, inflammatory reactions, cellular development, and diseases including hematopoiesis and others. Therefore, in respect of its diverse functions in immunity, leptin has been explored as a potential target for therapeutic development in treating autoimmune diseases.