As explained in the first part of the article, vitamins and trace elements influence various metabolic functions that are directly related to immune function. In this context, secosteroid vitamin D has met with growing interest. The discussion has focused on whether and, if so, to what extent, vitamin D might contribute to the prevention and possibly the treatment of infections and autoimmune diseases. We know, for instance, that immune cells are capable of synthesizing calcitriol from its precursor calcidiol, whereby the former enhances the synthesis of antibacterial peptides by macrophages while simultaneously inhibiting the (auto)immune response mediated by T helper cells (Th1). Numerous observational studies support the hypothesis that a vitamin D deficit increases the risk of autoimmune diseases such as type 1 diabetes mellitus, multiple sclerosis, psoriasis and rheumatoid arthritis; however, there are few reliable interventional studies to date. In general, immune status represents a sensitive indicator of micronutrient supply. Conversely, the activity of the immune system has an effect on the status of and requirements for nutrients.