CD3, a member of the immunoglobulin superfamily of cell surface receptors, is comprised of five invariable chains ranging from 16-28 kDa and is closely associated with the T cell antigen receptor (TCR). CD3 is expressed on 70-80% of normal peripheral blood lymphocytes and on 10-20% of thymocytes. It plays a major role in signaling during antigen recognition, leading to T cell activation. The monoclonal antibody UCHT1 reacts with the 20 kDa ? chain of the CD3/TCR complex.