1. （作出）反应；回应
    When you react to something that has happened to you, you behave in a particular way because of it.

    e.g. They reacted violently to the news...
           他们对这条新闻反应强烈。
    e.g. It's natural to react with disbelief if your child is accused of bullying...
           如果你的孩子被指责欺负了别的孩子，你不相信是很正常的。

2. 反抗;不服;反对
    If you react against someone's way of behaving, you deliberately behave in a different way because you do not like the way they behave.

    e.g. She reacted against the mindlessness and luxury of their lives...
           她对他们生活的盲目和奢侈很反对。
    e.g. My father never saved and perhaps I reacted against that.
           我父亲从来不存钱，我可能正好相反。

3. 产生不良反应;过敏
    If you react to a substance such as a drug, or to something you have touched, you are affected unpleasantly or made ill by it.

    e.g. 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。
    e.g. He reacted very badly to the radiation therapy.
           他接受放射治疗，副作用很大。

4. 起（化学）变化;发生（化学）反应
    When one chemical substance reacts with another, or when two chemical substances react, they combine chemically to form another substance.

    e.g. Calcium reacts with water...
           钙和水会发生化学反应
    e.g. Under normal circumstances, these two gases react readily to produce carbon dioxide and water.
           在正常情况下，这两种气体容易发生反应产生二氧化碳和水。