to behave in a particular way or show a particular emotion because of something that has happened or been said [↪ respond]
She reacted very badly (=was very upset) when her parents split up.
You have to react quickly to circumstances.
react by doing something➔ overreact
The government reacted by declaring all strikes illegal.
if a chemical substance reacts, it changes when it is mixed with another chemical substance
The calcium reacts with sulphur in the atmosphere.
if prices or financial markets react to something that happens, they increase or decrease in value because of it
Oil prices reacted sharply (=reacted a lot) to news of the crisis in the Middle East.
The market reacted favourably to the announcement.
to become ill when a chemical or drug goes into your body, or when you eat a particular kind of food [↪ respond]
Quite a lot of children react badly to antibiotics.
react against somethingphrasal verb
He reacted strongly against his religious upbringing.