CD3? is a 20kD peptide chain, which together with CD3?, CD3?, and CD3?, and a T cell receptor (?/? or ?/d) form the CD3/T-cell receptor complex. It is a specific marker for T lymphocytes, NK T cells, and some thymocytes. Crosslinking of TCR initiates an intracellular signaling cascade resulting in cellular activation and proliferation. The OKT3 antibody shows potent immunosuppressive properties in vivo and has been proved effective in the treatment of renal, heart, and liver allograft rejection.