Allen scored 10 of his points in a big third quarter and grabbed seven rebounds while Rajon Rondo distributed 18 assists to go with his 13 points as the Celtics bounced back from an opening loss in Cleveland to even the series as it heads to Boston for Game 3 on Friday.
Kevin Garnett contributed 18 points and 10 rebounds while Rasheed Wallace hit 7-of-8 field goals for 17 points off the bench in the Celtics' win.
The Cavs were held to only 12 points in the decisive third period and were awful from beyond the arc, connecting on only 4-of-21 three-point attempts.
LeBron James ended with 24 points and Antawn Jamison added 16 in Cleveland's loss. The Cavs suffered only their second playoff home defeat in their last 15 tries.