Selenium is essential to a healthy immune system although the details of how it works are quite complex. Suffice it to say that many of the selenium-containing proteins, referred to as selenoproteins, including glutathione reductase (GPx), thioredoxin reductase and selenoprotein P, to name just a few of the 25 selenoproteins in the human body, are essential for a healthy immune system. They play a role in both the humoral (ie, antibody producing) immune cells as well as those of the innate or cellular immunity, which are our first line of defense against viruses and cancer. Consequently when there is a selenium deficiency, viruses can mutate and become even more virulent.
One of the key ways that selenium helps with a robust healthy immune system is by maintaining glutathione levels. Glutathione (GSH) is one of the key pillars of a healthy immune system. If GSH levels are high, the immune system tends to function the way it should, but if low, then immune function suffers.