I was taught that the properties of plantain which make it helpful with bug bites and stings (and similar) when used externally are because it is astringent and antiphlogistic, so it reduces the inflammation and swelling, and that in the off season you
should make a decoction of the dried leaves for internal or external use, or a salve for external use.
I've heard people call it an antihistamine, but I've never seen a reputable source for that -- just the usual internet echo. If you have one, please share it with me; I have a life-threatening allergy as well. I'd rather rely on my Epi-Pen but if the plantain were fresh and in the right season I could see it opening the airways enough to get the reaction under control.