Leptin, a molecule that is critical in the regulation of energy balance, body weight, and reproductive function, is a strong regulator of T cell function.¹ This is one of many examples of redundancy and of the overlapping roles of molecules within the neuroendocrine and immune systems.² Leptin is part of the helical cytokine family along with interleukin (IL-) 6, IL-12, and IL-15, its receptor (ObR) belonging to the group of class I cytokine receptors, which includes gp-130, the common signal transducing component for the IL-6 related family of cytokines.¹ Leptin is expressed particularly in adipose tissue and to a lesser extent in other tissues such as muscle, stomach, and placenta.¹ More recently, leptin has also been shown to be expressed in activated inflammatory T helper 1 lymphocytes during experimental autoimmune encephalomyelitis, an animal model of multiple sclerosis.³ In keeping with these findings, the ObR has been found not only on the hypothalamus and adipose tissue but also on immune cells such as T lymphocytes and monocytes.^1,4 Addition of this hormone to T cells in culture can alter both their growth rate and pattern of production of cytokines—proteins that influence or mediate immune functions.^4,5 Indeed, leptin enhances the activity of T cells that produce proinflammatory cytokines and that orchestrate many …