735 ILCS 5/9-106
On complaint by the party or parties entitled to the possession of such premises being filed in the circuit court for the county where such premises are situated, stating that such party is entitled to the possession of such premises (describing the same with reasonable certainty), and that the defendant (naming the defendant) unlawfully withholds the possession thereof from him, her or them, the clerk of the court shall issue a summons.
The defendant may under a general denial of the allegations of the complaint offer in evidence any matter in defense of the action. Except as otherwise provided in Section 9-120, no matters not germane to the distinctive purpose of the proceeding shall be introduced by joinder, counterclaim or otherwise. However, a claim for rent may be joined in the complaint, and judgment may be entered for the amount of rent found due.
735 ILCS 5/9-106