QS-21

QS-21 (QS-21-Api; Stimulon) is a natural saponin vaccine adjuvant derived from Quillaja saponaria bark that enhances both Th2 humoral and Th1 cell-mediated immune responses by acting on antigen-presenting cells (APCs) and T cells. It activates the NLRP3 inflammasome, leading to caspase-1-dependent release of IL-1β and IL-18 cytokines, which are critical for Th1 immunity and control of intracellular pathogens.