CD137 antigen, a member of the tumor necrosis factor receptor family, and its ligand are reported to be expressed on activated T lymphocytes and on antigen presenting cells, respectively. This receptor/ligand system regulates the activation, proliferation and survival of T and B lymphocytes and monocytes through bidirectional signal transduction. Human CD137 antigen is reported to be expressed on activated B cells, Reed Sternberg cells and peripheral blood monocytes but is absent from resting T cells. In non-lymphoid cells, expression has been reported in blood vessel walls, on the endothelial layer and on vascular smooth muscle cells. Soluble forms of CD137 are reported at increased levels in sera of individuals with rheumatoid arthritis. The expression of soluble CD137 lags behind that of membrane bound CD137 by approximately 24 hours and it has been proposed that as activation of lymphocytes through membrane-bound CD137 delivers a potent stimulatory signal then soluble CD137 may provide a negative control mechanism for immune responses.