The human CD3/T cell receptor (TcR) complex is made up of at least five CD3 proteins (g, d, e, h, z) in association with either a /b or g/ d proteins of the TcR. The TcR recognizes antigens in association with MHC molecules after which protein chains of the CD3 complex mediate activation signals triggered by TcR antigen binding. CD3 is expressed on greater than 95% of circulating human peripheral T cells.