The CD3 antibody recognizes the 17-19 kD ε-chain of CD3 within the CD3 antigen/T cell antigen receptor (TCR) complex. The CD3 antigen is expressed in the cell cytoplasm during the early stage of T cell development and is expressed on the cell membrane at the late stage. CD3 antigen is displayed on 60-80% of normal peripheral blood lymphocytes and 60-70% of thymocytes and plays an important role in signal transduction after antigen recognition by TCR. NK cells also express the CD3 antigen in the cytoplasm.