The project consists of a placebo controlled selenium supplementation study and a dietary intervention with un-enriched and selenium enriched onions. In a parallel group design, subjects will be given either one of three doses of Selenomethionine (50, 100 or 200µg selenium/day) or a placebo per day or selenium enriched or un-enriched onions (in the form of test meals) for 12 weeks. Changes in the expression of Se-responsive genes and proteins in blood will be measured and compared with changes in plasma Se concentration and selected selenoproteins. The relationship between dietary Se intake and systemic and mucosal immune responses to influenza vaccine will be examined. Changes in immune cell populations and the influence of Se on NK and CD8 cytotoxicity will be determined by flow cytometry.