What to do about an allergic reaction without an epipen (epinephrine) at hand?

Don't delay! : If the allergic reaction is significant, anaphylactic, call 911. You need to get to emergency care immediately. This should be done even if you have and use an Epinephrine autoinjector. This is potentially an acute life threatening reaction. Antihistamines are of minimal help. The Epinephrine is to hopefully temporize symptoms till you get to an emergent care location. Don't delay!