Hi Katy, great question!
So elderberries are anti-viral, as well as rich in vitamin A and C to support the immune system. Both the flowers and berries are diaphoretic, with the flowers being more potent in this action as well as anti-catarrhal/decongestant. Because elderflowers are a mucus membrane tonic, especially in the upper respiratory tract, this helps increase resistance to infection. I honestly like to pair them together when somebody is acutely sick, and then use elderberry if one is trying to fight off colds/flu alongside some other immune boosting herbs.
Riley