Part of the TCR-CD3 complex present on T-lymphocyte cell surface that plays an essential role in adaptive immune response. 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. Upon TCR engagement, these motifs become phosphorylated by Src family protein tyrosine kinases LCK and FYN, resulting in the activation of downstream signaling pathways. In addition of this role of signal transduction in T-cell activation, CD3D plays an essential role in thymocyte differentiation. Indeed, participates in correct intracellular TCR-CD3 complex assembly and surface expression. In absence of a functional TCR-CD3 complex, thymocytes are unable to differentiate properly. Interacts with CD4 and CD8 and thus serves to establish a functional link between the TCR and coreceptors CD4 and CD8, which is needed for activation and positive selection of CD4 or CD8 T-cells. The TCR-CD3 complex is composed of a CD3D/CD3E and a CD3G/CD3E heterodimers that preferentially associate with TCRalpha and TCRbeta, respectively, to form TCRalpha/CD3E/CD3G and TCRbeta/CD3G/CD3E trimers. In turn, the hexamer interacts with CD3Z homodimer to form the TCR-CD3 complex. Alternatively, TCRalpha and TCRbeta can be replaced by TCRgamma and TCRdelta. Interacts with coreceptors CD4 and CD8. CD3D is mostly present on T-lymphocytes with its TCR-CD3 partners. Present also in fetal NK-cells. Phosphorylated on Tyr residues after T-cell receptor triggering by LCK in association with CD4/CD8.