The best scents for keeping your immune system strong? Lavender, frankincense, thyme, lemon, and tea tree oil are all known to boost immunity. These essential oils can be used alone or in combination- and are especially great to try when you are under a lot of stress or are feeling tired and rundown. It's easy: Simply add to a diffuser or bath (mix with 1 tsp. of almond or jojoba oil first for baths).