Heading to Italy w/ l70 min layover in London....immigration?

Probably not but we need a lot more information.

Is it one ticket or did you buy them separately?

Do you have check in luggage or just carry on?

Is it the same airline and terminal?

You can also go to http://www.heathrow.com/flight-connections and it will give you step by step instructions. But it may not be accurate depending on the answer to the above questions.

As long as it's just a connection, no. You should just stay airside. Check the Heathrow website regarding connections