205 ILCS 5/5c
A bank may acquire shares of stock of a bank or holding company which owns or controls such bank if the stock of such bank or company is owned exclusively (except to the extent directors' qualifying shares are required by law) by depository institutions or depository institution holding companies and such bank or company and all subsidiaries thereof are engaged exclusively in providing services to or for other financial institutions, their holding companies, and the officers, directors, and employees of such institutions and companies, and in providing services at the request of other financial institutions or their holding companies (also referred to as a "bankers' bank"). The bank may also provide products and services to its officers, directors, and employees. In no event shall the total amount of such stock held by a bank in such bank or holding company exceed 10 percent of its capital and surplus (including undivided profits) and in no event shall a bank acquire more than 15 percent of any class of voting securities of such bank or company.
205 ILCS 5/5c