40 ILCS 5/16-190
The right of a person to a retirement annuity or other benefit, to the return of contributions, the retirement annuity or other benefit itself, any optional benefit, any other right accrued or accruing to any person under the provisions of this Article, and the moneys in the fund created by this Article, shall be subject neither to attachment, garnishment, execution, or other seizure by process, nor to sale, pledge, mortgage or other alienation, and shall not be assignable except as in this Article provided. A person receiving an annuity or benefit may authorize withholding from such annuity or benefit for the purposes enumerated in the "State Salary and Annuity Withholding Act", approved August 21, 1961, as now or hereafter amended. The moneys in the fund are exempt from any state or municipal tax.
40 ILCS 5/16-190