When you react to something that has happened to you, you behave in a particular way because of it.

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.

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.

When one chemical substance reacts with another, or when two chemical substances react, they combine chemically to form another substance.