A murine IgG2a monoclonal antibody with immunosuppressive activity. Muromonab-CD3 binds to and inhibits CD3 on the surface of circulating T-lymphocytes; binding of muromonab-CD3 to CD3-positive T cells results in an early activation of this T cell subset, followed by cytokine release, and subsequently inhibition of T cell functions. This agent may cause the opsonization and elimination of CD3-positive T cells from the circulation by mononuclear phagocytes in the liver and spleen. CD3 is part of the functional T cell receptor (TCR) complex, which is necessary for antigen recognition by T cells, and is required for signal transduction or A type of monoclonal antibody used in cancer detection or therapy. Monoclonal antibodies are laboratory-produced substances that can locate and bind to cancer cells..