Selenium as an essential component of selenocysteine-containing protein is involved in most aspects of cell biochemistry and function. As such, there is much potential for selenium to influence the immune system. For example, the antioxidant glutathione peroxidases are likely to protect neutrophils from oxygen-derived radicals that are produced to kill ingested foreign organisms. When the functions of all selenoproteins are described, only then will it be possible to fully understand their role in maintaining optimal immune function.