respond

to react without any delay

to react fast

to react at once

to react quickly and without delay

to react in a good or helpful way

to react in a positive or approving way (BrE spelling)

to react in a positive or approving way (AmE spelling)

to react to someone in the same way that they have treated you

to react in a bad or unhelpful way

to react in a negative or harmful way

to react in an extremely good or impressive way

to reply in a courteous manner

to reply in a relaxed and composed way

having the skill or capacity to react

to react to something that has been said or done

to react by doing or providing something

used when a situation or condition does not improve after an action or treatment