The CD3 (cluster of differentiation 3) T cell co-receptor helps to activate both the cytotoxic T cell (CD8+ naive T cells) and also T helper cells (CD4+ naive T cells).
CD3 consists of a protein complex and is composed of four distinct chains.
The complex contains a CD3γ chain, a CD3δ chain, and two CD3ε chains.
These chains associate with the T-cell receptor (TCR) and the zeta-chain to generate an activation signal in T lymphocytes.
The TCR, ζ-chain, and CD3 molecules together constitute the TCR complex.
The CD3γ, CD3δ, and CD3ε chains are highly related cell-surface proteins of the immunoglobulin superfamily containing a single extracellular immunoglobulin domain.
Containing aspartate residues,
The transmembrane region of the CD3 chains contain aspartame residue, and is negatively charged, a characteristic that allows these chains to associate with the positively charged TCR chains.
Phosphorylation of CD3 renders the CD3 chain capable of binding an enzyme called ZAP70 kinase that is important in the signaling cascade of the T cell.
CD3 is required for T cell activation.
CD3 is expressed in the cytoplasm of pro-thymocytes, the stem cells from which T-cells arise in the thymus, and then differentiate into common thymocytes, and then into medullary thymocytes, and it is at this latter stage that CD3 antigen begins to migrate to the cell membrane.
The antigen is found bound to the membranes of all mature T-cells, and in virtually no other cell type.
It does appear to be present in small amounts in Purkinje cells.
This high specificity, combined with the presence of CD3 at all stages of T-cell development, makes it a useful immunohistochemical marker for T-cells in tissue sections.
The CD3 antigen remains present in almost all T-cell lymphomas and leukemias, and can therefore be used to distinguish them from superficially similar B-cell and myeloid neoplasms.