Cysteine is a nonessential amino acid (protein building block), meaning that cysteine can be made in the human body. Cysteine is one of the few amino acids that contains sulfur;. This allows cysteine to bond in a special way and maintain the structure of proteins in the body. Cysteine is a component of the antioxidant;glutathione. The body also uses cysteine to produce taurine , another amino acid. Most people do not need to supplement with cysteine. Almost nothing is known about appropriate supplemental levels, in part because almost all clinical research has been done with N-acetyl cysteine and not cysteine itself.The body can synthesize cysteine from methionine and other building blocks. Cysteine, the amino acid from which NAC is derived, is found in most high-protein foods.According to several studies, blood levels of cysteine and glutathione are low in people infected with HIV .1 , 2 , 3 Cysteine has a role in the proper function of the immune system , so a deficiency of this amino acid may either contribute to, or result from, immune suppression associated with HIV.