Usage patterns
襟を立てるto turn up one's collar
It was cold, so I turned up my collar and walked.
〈人〉の襟をつかむto grab someone by the collar
He got angry and grabbed the other person by the collar.
襟が汚れるfor a collar to get dirty
The collar of a white shirt gets dirty quickly.
襟を開けるto open (loosen) one's collar
It was hot, so I loosened my collar a little.
Expressions
襟を正すstraighten oneself; compose oneself; take a serious attitude