Through googling I found this:
N-Acetyl Cysteine (NAC) is the pre-acetylized form of the simple amino acid Cysteine. A powerful antioxidant, a premier antitoxin and immune support substance, and is found naturally in foods. It is a precursor for glutathione, an important antioxidant that protects cells against oxidative stress. In addition to maintaining intracellular glutathione levels, NAC supplementation has been shown to suppresses Human Immunodeficiency virus (HIV) replication, to be protective against cell damage caused by chemotherapy and radiation therapy, to be immune enhancing, to protect against toxins as acetametaphen and other drugs, mercury, lead, and others, and is mucolytic, that is, it breaks up mucus seen in bronchoulmonary disease as cystic fibrosis, chronic bronchitis, asthma, and pneumonia.