Amplification of FcγR-induced Syk activation by LTB₄ is receptor mediated. Rat alveolar macrophages were pretreated with LTB₄RA (LY 292476) at 1 μM for 10 minutes prior to the addition of LTB₄ (10 nM). Two minutes after LTB₄ treatment, the cells were challenged with IgG-SRBCs (1:33 ratio) and then incubated for 7 minutes at 37°C. Incubations were terminated by addition of lysis buffer, and lysates were subjected to immunoprecipitation and immunoblotting as described in “Materials and methods.” Immunoblots in upper panels represent phosphorylated Syk detected with antiphosphotyrosine antibody, and those in lower panels, the amounts of Syk protein evaluated with anti-Syk antibody. Results are representative of 2 separate experiments.