T cell activation is initiated by recognition of antigen by the T cell receptor (TCR) in the context of Class II MHC on an antigen-presenting cell. The T cell receptor contains multiple subunits and interacts with several factors to transduce antigen-stimulated T cell activation. One of the key steps that initiates receptor activation is the tyrosine phosphorylation of TCR subunits by the src family protein kinases Lck and Fyn. T cell receptor phosphorylated by Lck and Fyn recruits the ZAP-70 protein kinase to the receptor complex, which becomes activated and stimulates downstream pathways like the Map kinase cascade (see 'T Cell Receptor Signaling Pathway'). The CD45 protein tyrosine phosphatase activates Lck and Fyn by dephosphorylating these proteins and is required for TCR activation. Association of the CD4 coreceptor with Lck may also be involved in Lck activation.
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