Selenium is a trace mineral.
The highest selenium levels are found in the liver, kidney, heart, and spleen. Selenium's main function is as part of the antioxidant enzyme glutathione peroxidase, which protects the body from free-radical damage. Selenium works with vitamin E in the antioxidant defense system. The antioxidant role of selenium protects the heart from free radicals. A healthy immune system requires adequate levels of selenium, and moderate doses of supplemental selenium might enhance immunity.