Mich. Comp. Laws § 767.94: Immunity of witness.

Sec. 94.

If a person comes into this state in obedience to a summons which is issued pursuant to section 93 he shall not while in this state pursuant to such summons be subject to arrest or the service of civil or criminal process in connection with matters which arose before his entrance into this state under the summons.

If a person passes through this state while going to another state in obedience to a summons to attend and testify in that state or while returning therefrom, he shall not while so passing through this state be subject to arrest or the service of civil or criminal process in connection with matters which arose before his entrance into this state under the summons.