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. CD3Z ITAMs phosphorylation creates multiple docking sites for the protein kinase ZAP70 leading to ZAP70 phosphorylation and its conversion into a catalytically active enzyme. Plays an important role in intrathymic T-cell differentiation. Additionally, participates in the activity-dependent synapse formation of retinal ganglion cells (RGCs) in both the retina and dorsal lateral geniculate nucleus (dLGN). 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 SLA (PubMed:10662792). Interacts with SLA2 (PubMed:11891219). Interacts with TRAT1. Interacts with DOCK2. Interacts with SHB. Interacts with ZAP70. Interacts (tyrosine phosphorylated) with SHC1 (via SH2 domain). Interacts with PTPRC (By similarity). Interacts with CRK; this interaction regulates CD3Z phosphorylation (By similarity). Interacts (on T cell side) with CD81, ICAM1 and CD9 at immunological synapses between antigen-presenting cells and T cells. Interacts with CD160. Interacts with LY6E. Interacts with LY6E (By similarity). The signaling subunit of immunoglobulin gamma (IgG) Fc receptor complex. As a homodimer or a heterodimer with FCER1G, associates with the ligand binding subunit FCGR3A (via transmembrane domain); this interaction is a prerequisite for Fc receptor complex expression on the cell surface. CD3Z is expressed in normal lymphoid tissue and in peripheral blood mononuclear cells (PBMCs). Expressed also in retinal ganglion cells (PubMed:20188655). The ITAM domains mediate interaction with SHB. Phosphorylated on Tyr residues after T-cell receptor triggering by LCK in association with CD4/CD8. CD3Z deletion causes severely defective thymocyte differentiation (PubMed:8223495). Absence of CD3Z also leads to altered dendritic structure and motility in developing retina (PubMed:20188655). Belongs to the CD3Z/FCER1G family.