Natural killer cells are part of the body's innate, or first-line, immune response. They respond more quickly than the adaptive immune system comprised of B cells and T cells. Once activated, natural killer cell activity peaks in as quickly as several hours, providing rapid support and wide-range immune defense. EpiCor^® is a fermentate of Saccharomyces cerevisiae containing various bioactive metabolites. These nutrient metabolites are produced in a specialized fermentation process and enhance immune cell production. In one cellular study, EpiCor^® promoted natural killer cell activation, as well as that of B cells and T cells. In two randomized, double-blind, placebo-controlled trials, EpiCor^® provided statistically significant support for upper respiratory tract health and overall immune health. A separate open label, in-house trial indicated a strong potential for EpiCor^® supplementation to enhance secretory IgA levels after 30 days. These various actions demonstrate the ability of EpiCor^® to address both innate and adaptive immune responses. NAC (n-acetyl-l-cysteine) promotes endogenous glutathione production for antioxidant protection and provides lung defense mechanisms for healthy respiratory function. In a randomized double-blind trial involving mostly older individuals, NAC showed support for respiratory health. Quercetin and elderberry provide flavonoids that offer additional support for immune and respiratory function while vitamin C, vitamin D and zinc play important roles in immune cell activation.