Abstract: Plant secondary metabolites play a central role in mediating the therapeutic effects of medicinal plants. Among them, flavonoids, alkaloids, terpenoids, and saponins are widely recognized for their diverse pharmacological activities. This systematic review aims to critically evaluate published literature on the isolation, characterization, biological activities, and molecular mechanisms of these major phytochemical classes. Scientific databases including PubMed, Scopus, Web of Science, and ScienceDirect were systematically searched for relevant peer-reviewed articles. Eligible studies were screened based on predefined inclusion and exclusion criteria. The reviewed literature demonstrates that these phytochemicals modulate key molecular pathways associated with oxidative stress, inflammation, microbial resistance, and cellular protection. Evidence further suggests that synergistic interactions among these compounds contribute significantly to the efficacy of herbal formulations. This systematic review provides a consolidated framework linking phytochemical diversity to therapeutic mechanisms and highlights future research directions for natural product–based drug discovery.

Keywords: Flavonoids; Alkaloids; Terpenoids; Saponins; Bioactive compounds; Molecular mechanisms; Pharmacological activity.