a person's appearance (as of body, or style, or face, etc.) / the state or situation of a thing / the general trend, tenor, or feel of a thing / one's social station, status, or quality

not necessary to; don't have to

Expresses gratitude or relief for something that happened in the past, a choice made, etc.

If only ~

It's good / okay to... / is fine

transplant rejection / allergic reaction

chain reaction (series of events) / chain reaction (type of nuclear reaction)

everything

(chemistry) a chemical reaction

an overreaction