Glutathione is found in all cells in the body.

Glutathione is a small molecule made up of three amino acids, which exists in almost every cell of the body. However, Glutathione, must be generated within the cell from its precursors before it can work effectively in the body. The presence of glutathione is required to maintain the normal function of the immune system. It is known to play a critical role in the multiplication of lymphocytes (the cells that mediate specific immunity) which occurs in the development of an effective immune response.

While all cells in the human body are capable of synthesizing glutathione, liver glutathione synthesis has been shown to be essential. Following birth, mice with genetically-induced loss of GCLC (i.e., GSH synthesis) only in the liver die within 1 month of birth. The biosynthesis pathway for glutathione is found in some bacteria, like cyanobacteria and proteobacteria, but is missing in many other bacteria. Most eukaryotes synthesize glutathione, including humans, but some do not, such as Leguminosae, Entamoeba, and Giardia. The only archaea that make glutathione are halobacteria.

Glutathione maintains proper oxidation-reduction (redox) potential inside cells. Redox affects the oxidation state of sulfur in enzymes, and thus affects the rates of biochemical reactions in cells. It also scavenges peroxides and oxidizing free radicals directly and also serves as the basis for the antioxidant network. Glutathione stores and transports cysteine throughout the body.

Glutathione exists in reduced and oxidized states. In the reduced state, the thiol group of cysteine is able to donate a reducing equivalent to other unstable molecules, such as reactive oxygen species. In donating an electron, glutathione itself becomes reactive, but readily reacts with another reactive glutathione to form glutathione disulfide. Such a reaction is possible due to the relatively high concentration of glutathione in cells. GSH can be regenerated from GSSG by the enzyme glutathione reductase.