820 ILCS 140/5.5: Retaliation prohibited.

An employer, or agent or officer of an employer, violates this Act if he or she discharges, takes an adverse action against, or in any other manner discriminates against any employee because that employee has:

(1) exercised a right under this Act;

(2) made a complaint to his or her employer or to the Director or the Director's authorized representative;

(3) caused to be instituted or is about to cause to be instituted any proceeding under or related to this Act; or

(4) testified or is about to testify in an investigation or proceeding under this Act.