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CD3 (Cluster of Differentiation 3) is a complex of proteins that associates directly with the T cell antigen receptor (TCR). Antigen binding to the TCR leads to IL-2 secretion via activation of a tyrosine phosphorylation pathway and a phospholipase C (PLC) pathway, in turn activating protein kinase C. CD3 is composed of five invariant polypeptide chains that associate to form three dimers. The five invariant chains of CD3 are labeled gamma, delta, epsilon, zeta, and eta. The zeta chain plays an important role in coupling antigen recognition to several intracellular signal-transduction pathways. Loss of the zeta chain results in the synthesis of unstable TCRs. A decrease of CD3 zeta has been described in T cells from patients with cancer, lupus and chronic infectious diseases.
