Vitamin D3 is responsible for many functions of the immune system, such as activating T cells to kill viruses and other disease-causing agents and regulating the inflammation in the airways caused by colds and flu. NAC (N-Acetyl Cysteine) is one of the building blocks of glutathione, but it’s also an effective antioxidant on its own. NAC may also improve immune health, thin mucous, reduce airway inflammation, and help to make breathing easier.