Engagement of the TCR by peptide antigen, in association with MHC gene products, leads to a series of intracellular biochemical events culminating in the transcription of new genes and cellular activation. The earliest identifiable intracellular change documented at present is the activation of one or more tyrosine kinases that phosphorylate first the CD3 chains themselves and subsequently other substrates. Subsequent to tyrosine kinase activation a series of secondary events have been observed to follow TCR engagement, including activation of serine/threonine kinases, activation of the GTP-binding protein p21ras and activation of transcription factors for receptors and growth factors such as the major T cell growth factor interleukin-2 (IL2). The CD4 and CD8 co-receptors bind a tyrosine kinase (p56lck) via their intracytoplasmic tail which plays a critical role in T cell signalling.

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