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? 他對你的建議是什么反應？
They have different reactions to the political events they read about in the newspaper. 對于從報紙上讀到的政治事件，他們的反應不一。
Human reactions to body language are not always known in advance, but sometimes they are. 人對于形體語言的反映不總是可以預見的，但有時是可以預見的。
"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 鐵與氧的化學反應