Two models for α₂β₁ integrin–stimulated mast cell activation and cytokine secretion. Model 1: a 2-site, 2-receptor model in which concurrent activation of the α₂β₁ integrin and a second coreceptor (eg, FcRγ or TLR) stimulates mast cell activation. Model 2: complement activation results in the deposition of complement components onto immune complexes and release of C3a, C5a, or both. Simultaneous stimulation of the α₂β₁ integrin and complement receptors (CR1, CR3, CR4, C3aR, or C5aR) stimulate mast cell activation.