The T‐cell receptor (TCR)–CD3 complex, expressed on T cells, is responsible for the recognition of antigens bound to major histocompatibility complex (MHC) molecules and determines the outcome of T-cell responses. It helps to activate both the CD8+ cytotoxic T cells and the CD4+ T helper cells.
Each T cell contains a unique αβ T-cell receptor (TCR), which does not possess intracellular signaling domains itself. Instead the TCR is noncovalently associated with a multisubunit signaling apparatus, consisting of CD3γ, CD3δ chain, CD3ε chains and CD3ζζ-chains.
TCR/CD3 signaling is central to the initiation of antigen-specific T cell responses to pathogens and vaccines, as well as transplanted tissues, tumors, and autoantigens. CD3 is initially expressed in the cytoplasm of pro-thymocytes. During T cell maturation the expression of CD3 migrates to the cell-membrane. The specific appearance at all stages of T cell development make CD3 a useful immunohistochemical marker for T cells in tissue sections.
In the clinical setting, CD3 is a relevant marker for the classification of malignant lymphomas and leukemias as the antigen remains present in almost all T-cell lymphomas and leukemias. It can also be used to detect T cells in celiac disease, lymphocytic and collagenous colitis.