A purified, natural saponin isolated from the soapbark tree Quillaja saponaria Molina with potential immunoadjuvant activity. When co-administered with vaccine peptides, QS21 may increase total antitumoral vaccine-specific antibody responses and cytotoxic T-cell responses. or A substance from plants that is being studied together with vaccine therapy for its ability to improve the way the immune system responds to disease. It belongs to the family of drugs called saponin adjuvants.