Fig. 3. Outside-in signalling associated with integrins on leukocytes. The inactive β2 and β3 integrins on myeloid cells and platelets, respectively, are constitutively associated with inactive Src kinase. Src is maintained in a C-terminal phosphorylated conformation by Csk kinase. The ligation of integrin to ligand prompts the dephosphorylation of the inhibitory Tyr by phosphatases, such as PTP1B and CD45, dissociation of Csk and autophosphorylation onto the activation loop of Src. Active Src can then dually phosphorylate an ITAM-containing adaptor that has been postulated to be associated with integrin through a linker. The phosphorylated ITAM recruits Syk through its tandem SH2 domains (pale blue). Syk then associates with the integrin β-tail and is in sufficient proximity to Src to be phosphorylated. Active Syk then phosphorylates downstream effectors, such as Vav1, Vav3 and SLP-76.