What supplements can help the immune system get stronger?

Several. First, lifestyle! cut down on sugar, because sugar can weaken the immune system, increase inflammation. You need enough sleep, healthy diet, hydration, minimize stress. You can add bee propolis, oregano oil, vit c (bioflavanoids also help, so better to get your c from foods!) astragalus, elder flower. Maitake mushrooms (beta glucan) also promising.