Glutathione

Glutathione is a tripeptide, made of Glutamine, Cysteine and Glycine. It is mostly produced in the liver and can be found in high sulfur containing foods like cruciferous vegetables. It is well-known as one of the main antioxidants produced in the body for cellular detoxification and for reducing oxidative damage caused by free radicals. Glutathione is also a powerhouse anti-viral and anti-bacterial nutrient. It plays a crucial role in decreasing viral replication within the cell as well as cell to cell transmission within the body. Studies show that Glutathione enhances T-Cell activity in the immune system which is needed to coordinate action against viruses and bacteria. It also helps balance the immune system, reducing allergies and auto-immune activity. Research shows that lowered levels of cysteine and glutathione in the body correlates with lowered immunity and increased risk of infection.
The body's antioxidant defense systems can be overwhelmed because of improper nutrition lacking in antioxidant nutrients, toxin overload, and other stressors, such as poor sleep and exercise. In turn the immune system functions less optimally and you become more susceptible to viral or bacterial infections. Glutathione as a supplement aids in the reduction of this oxidative stress in the body and immune system. Supporting the immune system this way gives it it the "ammo" needed to help to fight off infections such as the flu virus.

https://www.ncbi.nlm.nih.gov/pubmed/15798812

https://www.ncbi.nlm.nih.gov/pubmed/11029606