Originally Posted by
hamster
RD is compatible with current 105 too. And brake levers are brake levers, there's nothing special about brake levers. So only the shift levers and the cassette are 9-speed specific.
The RD may work, but may not. For the STI's these won't work for 2 reason, 5500 (the last 9 speed 105) is 9 speed, and won't have enough clicks to shift a 10 speed cassette, and the ratio on the brakes pull has changed, they will still work, but using older and current Shimano STI's/brake calipers together gives less than optimal performance.