The adjective reactionary is a political term meaning very conservative. It also functions as a noun describing someone who is politically reactionary. If you need an adjective meaning (1) tending to react or (2) characterized by reaction, go with reactive.
Reactionary is the opposite of radical (though radical is often used to describe extremes of any political persuasion). It comes from post-Revolutionary France, where the réactionnaires sought restoration of the old, monarchical order. In modern politics, the word denotes a steadfast conservatism, sometimes with fascist or xenophobic undertones. It’s often pejorative.
Examples
Reactionary
Unbinding Brahms — a composer in whom progressive and reactionary tendencies clashed — can mean unchaining a monster. [Los Angeles Times]
This agenda preys on the fears and paranoia of shut-ins and suburbanites and reactionary voters who don’t take the time to process or examine larger, more convoluted issues. [transcribed in Globe and Mail]
We all know there are reactionaries and progressives in all parties. [Guardian]
What could be more ludicrous than the spectacle of young people embracing an old reactionary who wanted to repeal the New Deal? [The Nation]
Reactive
Chemicals in the products make the skin more sensitive to ultraviolet light by making our cells much more reactive. [Daily Mail]
Mr. Redford’s gradual transition out of acting seems to have come because of his growing distaste for what is an intrinsically reactive job. [Wall Street Journal]