react vs follow up on

react and follow up on both are verbs.

Word	Noun	Adjective	Verb	Adverb
react	No	No	Yes	No
follow up on	No	No	Yes	No

As verbs, follow up on is a hyponym of react; that is, follow up on is a word with a more specific, narrower meaning than react:

Other hyponyms of react include act on, pursue, buck, go against.

react (verb)	follow up on (verb)
undergo a chemical reaction; react with another substance under certain conditions	carry further or advance
show a response or a reaction to something
act against or in opposition to