Selenium affects all components of the immune system, according to the Textbook of Natural Medicine by Joseph E. Pizzomo and Michael T. Murray.
Selenium deficiency results in depression of the immune function, whereas selenium supplementation results in augmentation and/or restoration of immune functions. The ability of selenium supplementation to enhance immunity goes beyond simply restoring selenium levels. In one study, they said, supplementation of 200 mcg/day to those with normal levels resulted in a 118% improvement in the ability of lymphocytes to kill tumor cells and an 82.3% rise in the activity of natural killer cells.
The results indicated that the immune-enhancing effects of selenium in humans require supplementation above the normal dietary intake.