Quil-A is a highly purified and concentrated saponin adjuvant purified from bark extract of the Quillaja saponaria Molina tree, which has been specifically developed for use as an adjuvant in vaccines. It consists of a complex mixture of approximately 25 different saponin molecules which have the triterpenoid backbone in common. Quil-A is a very versatile adjuvant due to its ability to induce a balanced immune response to protect against both intracellular (Th1) and extracellular (Th2) pathogens. When combined under specific circumstances with cholesterol, phosphatidylcholine, and amphipathic antigen, saponins can form stable immune stimulating complexes (ISCOMs). These complexes are spherical, nano-scale cage-like structures, which display immune-stimulatory properties. ISCOMs can be used to induce a stronger and longer-lasting immune response.