4-1BB is a member of the tumor necrosis factor (TNF) receptor family. It is currently of interest to immunologists as a co-stimulatory immune checkpoint molecule. It can be expressed by activated T cells, but to a larger extent on CD8 than on CD4 T cells. In addition, 4-1BB 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. The best characterized activity of 4-1BB is its costimulatory activity for activated T cells. Crosslinking of 4-1BB enhances T cell proliferation, IL-2 secretion, survival and cytolytic activity.