Commercially reared food animals encounter serial pathogenic and nonpathogenic immune challenges throughout production. Because of the diversion of nutrients away from growth in support of immune-related processes, immune challenge is considered a major obstacle to animals' achieving their genetic potential for growth or efficiency of gain. Scientists now recognize that many metabolic processes respond directly or indirectly to proinflammatory cytokines. This cytokine-mediated "reprogramming" of metabolism is a homeorhetic mechanism that ensures an adequate supply of nutrients for proliferation of lymphocyte and macrophage populations, antibody production, and hepatic synthesis of acute phase proteins. Proinflammatory cytokines have been linked to altered nutrient uptake and utilization. Anabolic processes are interrupted, and companion catabolic activities are amplified. Furthermore, cytokines may influence prenatal growth and development, and to the extent that postnatal proliferation and differentiation of myogenic and adipogenic cells contribute to postnatal growth, cytokine regulation of these events may ultimately influence growth. The following discussion is an overview of the impact of immune challenge and proinflammatory cytokines on metabolism and growth