4-1BB, is also known as CD137, is a type 2 transmembrane glycoprotein receptor belonging to the TNF superfamily.CD137 can be expressed by activated T cells, but to a larger extent on CD8 than on CD4 T cells. In addition, CD137 expression is found on dendritic cells, B cells, follicular dendritic cells, natural killer cells, granulocytes and cells of blood vessel walls at sites of inflammation.