735 ILCS 5/2-211

Service on public, municipal, governmental and quasi-municipal corporations. In actions against public, municipal, governmental or quasi-municipal corporations, summons may be served by leaving a copy with the chairperson of the county board or county clerk in the case of a county, with the mayor or city clerk in the case of a city, with the president of the board of trustees or village clerk in the case of a village, with the supervisor or town clerk in the case of a town, and with the president or clerk or other officer corresponding thereto in the case of any other public, municipal, governmental or quasi-municipal corporation or body.