QS-21 is a purified plant extract that enhances the ability of the immune system to respond to vaccine antigens. It is derived from the soap bark tree (Quillaja saponaria), which is native to the country of Chile.
The extract contains water soluble triterpene glucoside compounds, which are members of a family of plant-based compounds called saponins.
It is currently being tested as an immunologic adjuvant in various vaccines in attempts to improve their efficacy. It is believed to enhance both humoral and cell-mediated immunity.
QS-21 is currently under clinical evaluation as an additive for various trial vaccines, including those for HIV, malaria and cancer. As of 2002, it had been tested in more than 3000 patients in 60 clinical trials.
References
QS21 Wikipedia(Text) CC BY-SA