Selenium is a mineral and antioxidant (a warrior against harmful free radicals) that is essential for immune function. It helps produce new immune cells and can help to strengthen the response to infection.
Plus, it is involved in the production of antibodies (proteins made by the immune system that aid in defending against infection). Antibodies help to destroy harmful germs by quickly eliminating them or by preventing them from infecting cells.
So you can see how important this mineral is for immunity, and deficiency has been shown to impair aspects of both innate immunity (the immune system we are born with at birth) and adaptive immunity (can provide long-lasting protection sometimes for an entire lifetime, such as someone who contracts measles, and is now protected for their lifetime).
Additionally, selenium deficiency has been reported to increase the severity or progression of some viral infections (1).
How much selenium do we need a day?
In the UK, government guidelines for men and women aged 19-64 is 75mcg and 60mcg per day, respectively. If you eat meat, fish or nuts regularly you should be able to get all the selenium you need from your daily diet.
What food can selenium be found in?
In addition to meat and fish selenium can be found in Brazil nuts, sesame seeds, wholegrains, and seafood.
Should you supplement?
Whether to supplement depends entirely on your individual circumstances. Supplementation has been reported to enhance cell-mediated immunity in those who may be deficient, and also strengthen the response to viruses. In contrast, supplementation has been shown to exacerbate allergic asthma, so if you have allergic asthma I would recommend avoiding supplementing with selenium.
References
(1) Kiremidjian-Schumacher L, Roy M, Wishe HI, Cohen MW, Stotzky G. Supplementation with selenium and human immune cell functions. II. Effect on cytotoxic lymphocytes and natural killer cells. Biol Trace Elem Res. 1994 Apr-May;41(1-2):115-27.