Schematic model of αIIbβ3/FcγRIIa signaling in platelets. (A) Top view. (B) Side view. The integrin, which outnumbers FcγRIIa by more than 20:1, is shown in its bent, inactive conformation, which is still capable of binding immobilized, multivalent fibrinogen molecules. Ligand-binding–induced cross-linking of adjacent integrins leads to trans-activation of integrin β-subunit cytoplasmic domain-associated Src family kinases, a subset of which happen to be proximal to the cytoplasmic domain of FcγRIIa ITAM tyrosine residues. Their phosphorylation creates a docking site for the tyrosine kinase Syk, thereby completing transmission of the activation signals initiated by fibrinogen binding to the extracellular domain of αIIbβ3.