Maryland Code, Courts and Judicial Proceedings § 6-305

(a) A nonresident person who is within the State for the purpose of testifying in or prosecuting or defending an action may not be served with process.

(b) A nonresident person passing through this State to or from another state in response to a summons to testify may not be served with process. This subsection does not apply to process arising from acts done by the person after his entry into this State.