'unexpected reaction' is correct and usable in written English.
You can use it when something occurs that was not anticipated. For example: The moviegoers had an unexpected reaction to the ending of the film.
The images provoked an unexpected reaction.
"It was a completely unexpected reaction," recalled Mr. Bielec.
And there was another, unexpected, reaction to her work.
Ms. Morrison also sees a subtle, unexpected reaction.
An unexpected reaction was Wednesday's wave of anti-Muslim and anti-Iraqi sentiment.
The catfish case also provoked an unexpected reaction farther north, in the 125-year-old herring-canning industry of Maine.
One unexpected reaction commuting couples encounter is the assumption that there is some underlying problem in the marriage.
Users might write on a company's Wall about a specific product causing an unexpected reaction or injury.
"I'm probably older than you think," she said, an unexpected reaction from an author who has been so revealing.
It's an unexpected reaction to one of Perry's low-budget comedies that mostly trades in pratfalls and bawdy invective before slamming into a wall of treacly pop-psychology.
