CD137, a member of the tumor necrosis factor (TNF) receptor family, is a type 1 transmembrane protein, expressed on surfaces of leukocytes and non-immune cells. CD137 is only expressed on the cell surface after T cell activation. When T cells are activated by Antigen Presenting Cells (APCs), CD137 becomes embedded in CD4+ and CD8+ T cells. CD137 is a costimulatory molecule functioning to stimulate T cell proliferation, dendritic cell maturation, and promotion of B cell antibody secretion. As a T cell co-stimulator, T cell receptor (TCR) and CD28 signaling causes expression of CD137 on T cell membranes. When CD137 then reacts with the CD137 ligand, it leads to CD137 upregulation. This is a form of self regulation or positive feedback cycle. When CD137 interacts with its ligand, it leads to T cell cytokine production and T cell proliferation, among other signaling pathway responses.