Vitamin E works in cell membrane fractions. Selenium has its effect in the cystosol fraction. Thus, the functions of selenium and vitamin E are independent and additive. One of the main physiological changes brought by stress is elevation of serum glucocorticoid levels, which has been shown to reduce lymphocyte and antibody production and functions. Also, greater epinephrine secretions lead to higher levels of prostaglandin, lipoxin and other fat-based immunosuppressant enzymes.
Stress also leads to the release of arachidonic acid from the membrane phospholipids.
Once released, arachidonic acid breaks down into immune function inhibitors. White blood cells important in fighting disease are particularly susceptible to destruction by ROMs because of their high levels of free fatty acids. Since the late 1950s, vitamin A has been recognized as a limiting vitamin in ruminant diets, but less was known about vitamin E requirements until the 1980s. Vitamin E supplementation enhanced immune responses in other species. However, since green forages are naturally high in vitamin E, the NRC listed no requirements for cattle until 1984, and these are in a wide range.