QS-21 is a purified soapbark tree (Quillaja saponaria) extract that enhances the ability of the immune system to respond to vaccine antigens. QS-21 is a promising adjuvant candidate for use in humans due to the ease of purification, its improved safety profile, and its ability to enhance cellular and humoral immunogenicity. The mechanism of action of QS-21 was speculated to be similar to QA, forming complexes with cholesterol that intercalate into the cell membrane lipids. This intercalation creates pores in the membrane to accelerate the uptake of a co-delivered antigen by the antigen presenting cells. Multiple clinical trials using QS-21 as an adjuvant, demonstrated satisfactory safety profiles and enhanced immunogenicity in immunocompromised volunteers