An adequate host defense activity critically depends upon the micronutrient status of an individual among which the cellular oxidant-antioxidant balance is an important determinant. Oxidative burst is part of the physiological function of phagocytes connected to a massive production and release of reactive oxygen intermediates. At the same time, maintenance of the functional capacity of the host defense system in fighting against microorganisms and foreign antigens is significantly affected by the various reactive oxygen species. In order to compensate for this critical condition phagocytes do show active uptake and physiologically high intracellular concentrations of antioxidants. Thus, optimal function of the host defense system depends upon an adequate supply of antioxidative micronutrients and, on the other hand, impaired host defense activity can act as a very early and sensitive marker of marginal deficiency of antioxidant micronutrients. Assessment of immune functions can serve as an important preventative diagnostic tool in the detection of marginal but functionally relevant micronutrient deficiencies. Intervention into the functionally deficient antioxidant micronutrient status can act as the appropriate preventative or therapeutic treatment with higher efficacy and less adverse effects than direct pharmacological modulation of single immune functions by specific mediators.