To do something as a response meant to counter or thwart someone or something. It seems like the economy is reacting against the sudden overabundance of natural oil that has seen fuel prices plummet. We are not simply reacting against the senator nor his policies.

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.