alpha-Tocopherol is a form of vitamin E that is preferentially absorbed and accumulated in humans. The measurement of "vitamin E" activity in international units (IU) was based on fertility enhancement by the prevention of spontaneous abortions in pregnant rats relative to alpha-tocopherol. Vitamin E is an antioxidant responsible for proper functioning of the immune system and for maintaining healthy eyes and skin. It is actually a group of fat soluble compounds known as tocopherols. Vitamin E is a generic term for tocopherols and tocotrienols. Vitamin E is a family of alpha, beta, gamma, and delta; tocopherols and corresponding four tocotrienols. Vitamin E is a fat-soluble antioxidant that stops the production of reactive oxygen species formed when fat undergoes oxidation. Of these, alpha-tocopherol has been most studied as it has the highest bioavailability.