Schematic representation of proposed mechanism by which LTB₄ amplifies FcγR-mediated phagocytosis. (A) Resting cell is characterized by unoccupied FcγR and BLT, low [Ca²⁺], and inactive Syk and 5-LO. (B) Upon FcγR cross-linking by an IgG-coated particle, tyrosine residues in the cytoplasmatic tail of the FcγR become phosphorylated, creating a docking site for Syk, which in turn becomes tyrosine phosphorylated (activated). Syk then initiates a variety of other signaling events, culminating in increase of [Ca²⁺], 5-LO activation with LTB₄ synthesis, and phagocytosis. Endogenously produced or exogenously added LTB₄ interacts with BLT receptors. This further increases [Ca²⁺], which amplifies Syk activation as well as phagocytosis.