What is an immune system suppressing drug?

There are many: There are many immune suppressing drugs used for various reasons. The most common one is prednisone.

Many: Generically, a drug that interferes with immune system function. Specifically: mycophenolate mofetil, cyclosporine, methotrexate, azathioprine, etc. Today there are a host of monoclonal antibodies with more specific actions on the immune system. These meds all end in "mab" - e.G rituximab.