The immune system is a complex defense network that plays a crucial role in protecting the organism against infections, malignancies, and environmental stressors. In recent years, bioactive compounds derived from medicinal plants have attracted increasing attention in both complementary medicine and modern pharmacology due to their immunomodulatory properties. This review comprehensively examines medicinal plants that influence the immune system in terms of cultivation conditions, ecological factors, phytochemical composition, and molecular mechanisms of action. Furthermore, major phytochemical classes such as flavonoids, polysaccharides, terpenoids, and alkaloids are discussed with respect to their immunological activities. By integrating experimental and clinical studies published between 2020 and 2025, this review aims to provide a scientifically grounded assessment of the therapeutic potential of plant-based immunomodulators.