the audience reacted quickly to speech中为什么有个to

The audience reacted quickly to speech.
观众迅速对演讲作出反应。
react 下面这个意思进，是不及物动词，所以用to
vi.反应，作出反应； 影响； 起反作用
1. There'dbe no telling how John would react to such news as this.
还不知道约翰听到这样的消息会作何反应。
2. Someone allergic to milk is likely to react to cheese.
对牛奶过敏的人可能对奶酪也有不良反应。