react to (someone or something)
To do something as a response to the something or someone's actions. Our country will react swiftly and mercilessly to this outrage. My mother reacted very strongly to the news that my brother had been killed overseas. Don't react to him, Tom. He's just trying to get your attention, the way all toddlers do.
react to someone or something
to act in response to someone or something. You made some very good points. I would like to take some time to react to you. How did Mary react to the news?