The 3H3 monoclonal antibody reacts with mouse 4-1BB, a TNF receptor superfamily member also known as CD137. 4-1BB is a 39 kDa transmembrane protein expressed by T lymphocytes, NK cells, dendritic cells, granulocytes, and mast cells. Upon binding its ligand 4-1BBL, 4-1BB provides costimulatory signals to both CD4 and CD8 T cells through the activation of NF-κB, c-Jun and p38 downstream pathways. The importance of the 4-1BB pathway has been underscored in a number of diseases, including cancer. Agonistic anti-4-1BB antibodies have been reported to induce T cell mediated antitumor immunity. The 3H3 antibody is an agonistic antibody that has been shown to stimulate 4-1BB signaling in vivo.