This type of Qi is often referred to as “defensive Qi.” Its functions are equivalent to functions that Western science attributes to the immune system. Wei Qi is derived from Gu Qi, and is dispersed throughout the body by the Lung. Wei Qi is thought to gather at the surface of the body to fight off External Pathogenic Factors. When Wei Qi is weak, External Pathogenic Factors can invade into deeper levels of the body.