40 ILCS 5/6-212

Whenever any territory is annexed to a city, any person then regularly employed as a paid fireman in the annexed territory, who is employed as a fireman by the city on the date of annexation, shall automatically come under the provisions of this Article. Service as a fireman rendered in such territory shall be considered, for the purposes of this Article, as service rendered in such city. Any such fireman shall be treated, as of the date when such annexation shall come into effect, in the manner specified in this Article concerning present employees or future entrants of the city on the date upon which this Article shall come into force and effect in such city.