820 ILCS 191/20
An employer shall not deny an employee the right to use personal sick leave benefits in accordance with this Act or discharge, threaten to discharge, demote, suspend, or in any manner discriminate against an employee for using personal sick leave benefits, attempting to exercise the right to use personal sick leave benefits, filing a complaint with the Illinois Department of Labor or alleging a violation of this Act, cooperating in an investigation or prosecution of an alleged violation of this Act, or opposing any policy or practice or act that is prohibited by this Act. Nothing in this Section prohibits an employer from applying the terms and conditions set forth in the employment benefit plan or paid time off policy applicable to personal sick leave benefits.
820 ILCS 191/20