The immune responses to many infections have long been known to share features with autoimmune responses. In particular, both types of response are typified by the enhanced reactivity of T helper 1 cells - with high levels of interleukin-2, interferon gamma and tumor necrosis factor alpha - and are accompanied often by organ-specific and/or systemic damage and the production of IgG. Paradoxically, the geographical distributions of incidence of infectious diseases and autoimmunity are complementary, rather than coincident. In less-developed societies, an epidemiological association between susceptibility to infection and malnutrition has been observed, whereas in affluent countries, an increased incidence of autoimmune diseases has been described. We suggest that these observations can be explained partly by taking into consideration the immune effects of the adipocyte-derived hormone leptin, which has been shown recently to act as a link between nutritional status and the immune response.