<unk>  is a co-receptor of the T cell receptor (TCR) and assists the latter in communicating with antigen-presenting cells. The TCR complex and<unk>  bind to distinct regions of the antigen-presenting MHC class II molecule. The extracellular D1 domain of<unk>  binds to the β2 region of MHC class II. The resulting close proximity between the TCR complex and<unk>  allows the tyrosine kinase Lck bound to the cytoplasmic tail of<unk>  to phosphorylate tyrosine residues of immunoreceptor tyrosine activation motifs (ITAMs) on the cytoplasmic domains of CD3 to amplify the signal generated by the TCR. Phosphorylated ITAMs on CD3 recruit and activate SH2 domain-containing protein tyrosine kinases (PTK), such as ZAP70, to further mediate downstream signalling through tyrosine phosphorylation. These signals lead to the activation of transcription factors, including NF-κB, NFAT, AP-1, to promote T cell activation. Other interactions<unk>  has also been shown to interact with SPG21, and Uncoordinated-119 (Unc-119).
CD4
CD8
CD74
CD86
CD80
CD137
CD22
CDV3 