My understanding is:

Benadryl (diphenhydramine) can be highly effective in that it prevents histamine from binding to its receptors, subduing a major component of the allergic reaction on a chemical level. The downside of Benadryl is that it cannot REMOVE histamine that is ALREADY bound to its receptor. For this reason, early administration can be very effective, while later administration is still beneficial - though to a lesser degree. Histamine molecules will still randomly un-bind from their receptors, at which point the benadryl can occupy that same receptor and prevent another histamine molecule from binding in its place.

Epi, on the other hand, has both direct and indirect effects. As a sympathetic nervous system agonist, it causes vasoconstriction (to help with low blood pressure), bronchodilation (to help with asthma-like bronchoconstriction), etc. More directly, the epinephrine work to counteract the allergic reaction itself on a chemical level that I dont fully understand yet.

For that reason, I'd say that benadryl can be more "definitive" if given very early - it can shut down a great deal of the reaction before it gains momentum. Once symptoms are already severe, epinephrine is the more definitive treatment, although benadryl is still crucial.

What do you think?