The saponins are a subclass of terpenoids, the largest class of plant extracts. The amphipathic nature of saponins gives them activity as surfactants with potential ability to interact with cell membrane components, such as cholesterol and phospholipids, possibly making saponins useful for development of cosmetics and drugs. Saponins have also been used as adjuvants in development of vaccines, such as Quil A, an extract from the bark of "Quillaja saponaria". This makes them of interest for possible use in subunit vaccines and vaccines directed against intracellular pathogens. In their use as adjuvants for manufacturing vaccines, toxicity associated with sterol complexation remains a concern.