Advances in the understanding of the interrelationship between immunology and nutrition indicate that immune dysfunction in critically ill patients is linked with nutrient deficiency and abnormal acute-phase response to illness. Immune dysfunction requires special nutrition therapy and metabolic support; immunoregulation by nutrition manipulation may lead to specific immunotherapies for defined groups of patients. The success of dietary strategies must be measured not only by metabolic indexes but also by effects on immune function. The health profession must combat immune dysfunction and inflammation for the sick and frail patients under its care.