The CD3 antigen is present on mature human T cells, thymocytes, and NKT cells. It is associated with the T cell receptor (TCR) and is responsible for the signal transduction of the TCR. The CD3 antigen is a complex of five invariable chains: γ, δ, ε, ζ, and η. The CD3 antibody recognizes all T cells, i.e., it reacts with 70–80% of human peripheral blood lymphocytes and with 65–85% of thymocytes. The epitope recognized by the antibody is located on the ε-chain of the CD3 complex.