Immunoadjuvants have major effects on the immune responses in vaccine formulations. Many specific adjuvants have been studied, including mineral salts, emulsions, cytokines, non-ionic block copolymers, carbohydrate polymers, muramyl dipeptides, and saponin-based adjuvants. Here, we focus on quillaic acid-containing saponin-based adjuvants and their mechanisms of action. Interestingly, when analyzing the adjuvants capable of forming immune-stimulating complexes, one of the characteristics shared by the majority of them is the presence of quillaic acid as the aglycone. This suggests that quillaic acid might be a strategic molecule for immunoadjuvant activity and a powerful candidate in the development of novel adjuvants.