T-cell surface glycoprotein CD3 epsilon CD3 delta chain also known as CD3E CD3D are single-pass type I membrane proteins When antigen presenting cells (APCs) activate T-cell receptor (TCR) TCR-mediated signals are transmitted across the cell membrane by the CD3 chains CD3D CD3E CD3G and CD3Z All CD3 chains contain immunoreceptor tyrosine-based activation motifs (ITAMs) in their cytoplasmic domain