1) Created an opposing response 2) Did a double take 3) Did a double-take 4) Did a knee jerk 5) Flinched 6) Flinched or blinked 7) Lashed back 8) Responded to 9) Responded to a stimulus 10) Showed a response 11) Showed an effect 12) Underwent chemical change Found on https://www.crosswordclues.com/clue/reacted