QS-21 is a purified Quillaja Saponaria extract that enhances the ability of the immune system to respond to vaccine antigens. It can activate key components of the immune system including cellular (T cell mediated) and humoral (antibody mediated) immunity. Both of these components of the immune system are often necessary to enable the body to heal or protect itself. It is currently a key adjuvant component in 20 clinical development programs.