The immune system protects the body against pathogens and cancer cells, thereby defending it against infections and diseases via three main components: epithelial barriers, immune cells and proteins including antibodies. Immune defences are constantly active and consist of a complex network of coordinated biological responses. Therefore, the immune system needs to be fed properly with energy sources and essential micronutrients serving as cofactors in the development, maintenance and expression of the immune response. The general nutritional status of an individual modulates immune functions, and immunocompetence is regarded as a measure of adequate nutrition. Low levels of vitamins, minerals and trace elements lead to suppressed immunity, predisposing individuals to infections, which in turn aggravate the nutritional status, leading to a vicious cycle. Supplying the deficient micronutrients with the diet or a dietary supplement can re-establish immune function. A sufficient and balanced diet should cover the overall micronutrient requirements. However, many population segments both in developing and industrialized countries do not get adequate amounts of essential micronutrients through the diet. Current environmental and geopolitical developments (i.e. global economic crisis, rice crisis and other food shortages, the increase in maize cultivation for ethanol production, etc.) are exacerbating problems of nutritional status in many parts of the world and represent a serious threat to nutritional health. This review focuses on nutritional aspects of the immune system and discusses the roles of vitamins A, D, E, C, B6, B12 and folate as well as of the trace elements selenium, zinc, iron and copper in supporting the body's natural defence system and restoring resistance to infections by enhancing the three levels of immunity: epithelial barriers, immune cells and antibody production.