N-acetyl cysteine (NAC) is a stable form of the non-essential amino acid cysteine. It is a sulfur-containing amino acid that acts as a stabilizer for the formation of protein structures, and promotes the formation of glutathione. Glutathione is a powerful free radical scavenging compound that also helps to maintain normal, balanced immune system function. Quercetin is a naturally occurring free radical neutralizer that supports healthy seasonal immune system function. Laboratory studies have demonstrated that quercetin can also help to promote normal respiratory function. Zinc has been included for its immune-supporting properties.