Modified polysaccharides from snow fungus (Tremella fuciformis) can directly fortify immune cells against oxidative stress rather than acting as passive antioxidants. In macrophage cultures, the phosphorylated forms most strongly activated cellular defense pathways, sharply increasing antioxidant capacity, boosting nitric oxide signaling, and reducing free-radical–induced damage compared with unmodified sugars. The results indicate snow fungus polysaccharides can actively upregulate immune resilience and stress resistance at the cellular level, helping explain their long-standing use for skin protection, immune support, and healthy aging.