T cells are a category of lymphocytes that differ from other lymphocytes because of the unique T cell receptor (TCR) present on the cell surface. Together with co-receptors (CD4 for helper T cells and CD8 for cytotoxic T cells), the TCR enables T cells to detect and react to peptide antigens, which are pathogens digested by antigen presenting cells (APCs) and displayed on major histocompatibility complex (MHC) molecules.
Upon TCR engagement, Src kinases lymphocyte-specific protein tyrosine kinase (Lck) and Fyn phosphorylate immunoreceptor tyrosine-based activation motifs (ITAMs) of the TCR/CD3 complex. ZAP70 (zeta-chain-associated protein kinase 70) is recruited to amplify the response.
Through protein tyrosine phosphorylation, several pathways can be triggered, such as the ERK, JNK, NF-κB, and NFAT pathways. Stimulation of the TCR will launch positive and negative cascades that impact T cell activation, proliferation, differentiation, cytokine production, and apoptosis.
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