Clearing immigration in Europe?

I am going to Prague from the US. On the way home, my flight leaves Prague and has a layover in Frankfurt and London. Since I will be starting my flight in Europe, do I have to clear immigration in either of these places before I land in the US? I have a 1:15 layover in Frankfurt and a 2:40 in London. I think I will be ok if I don't have to clear immigration. No checked bags.