Notice a person's reaction to stress in these situations and to aggressive behavior in others. (Para. 6)
reaction: the way a person reacts to something
reaction to: reply to; something produced by ...
What was his reaction to your suggestion? 他对你的建议是什么反应？
They have different reactions to the political events they read about in the newspaper. 对于从报纸上读到的政治事件，他们的反应不一。
Human reactions to body language are not always known in advance, but sometimes they are. 人对于形体语言的反映不总是可以预见的，但有时是可以预见的。
"Reaction" also means "a course of change that occurs when two or more kinds of matter are put together".
a chemical reaction between iron and oxygen 铁与氧的化学反应