Adequacy or deficiency in the micronutrient status of an individual deeply affects its immunocompetence. Among the different micronutrients, antioxidative vitamins and beta-carotene are particularly effective in modulating immune functions and host defense against microorganisms or other invasive processes. This involves antigen-specific humoral or cell-mediated reactions of the immune system as well as nonspecific inflammatory processes. Owing to the interactions between micronutrient status and immunocompetence, the assessment of the micronutrient status of an individual can help to predict the risk of specific diseases associated with a deficient or suppressed immunocompetence, eg, infection or cancer. By the appropriate intervention it should be possible to modify this risk as has been shown in a number of carefully controlled studies. Assessment of the immunocompetence of an individual by measuring a limited number of key parameters of the immune system can help in the very early detection of specific micronutrient deficiencies.