 Selenium Se is a trace element that is an essential component of several metabolic pathways controlling immune function. Se and As work antagonistically in the body by competing in many biological functions. Arsenic added to animal diets has been known to counteract Se toxicityactivity in animals since the 1930s. Se interacts with As to form a complex; excessive Se excretion can occur as result of SeAs complex formation under condition of chronic As ingestion and low dietary Se ingestion. This scenario may further accelerate Se depletion. Again, since Se can form a complex with As, Se supplementation, in principle, can reduce As toxicity and also decrease effects of As exposure on many other parameters, such as oxidative stress, immunotoxicity etc. 