Early events in T cell receptor signal transduction

After antigen binds the TCR, Lck is activated and phosphorylates tryosine residues within the ITAMs of the CD3 and zeta chains, allowing ZAP-70 to be recruited to the TCR. Once ZAP-70 binds the phosphorylated ITAMs, it is phosphorylated by Lck, triggering all downstream signaling events required for T cell activation.