Originally Posted by 13901

If your trip is on a single ticket your bag should be checked through from AA in 3 to BA in 5. They drive a cart with the transfers from AA to terminal 5 and then the various handlers come and pick up theirs. Having seen it in the flesh it's all very, aehm, rustic compared to Heathrow.

The pick-up-and-re-check ballet is only for the other way round, i.e. coming from outside the US and into the country.