Mechanism of collaboration of the C3 receptor (a β₂ integrin) and the FcγIII receptor in the activation of the leukocyte NADPH oxidase.95 In the resting membrane, the C3, FcIIγ, and FcIIIγ receptors are empty. (1) Occupancy of the C3 receptor causes the FcIIγ receptor to bind to the cytoskeleton. (2) Occupancy of the FcIIIγ receptor activates a tyrosine kinase. (3) The activated tyrosine kinase phosphorylates the cytoplasmic portion of the FcIIγ receptor, which was rendered susceptible to phosphorylation through its interaction with the cytoskeleton. (4) The phosphorylated FcIIγ receptor activates the oxidase via a multistep signal transduction pathway.