Antibody–antigen complexes are central to the inflammatory response and are implicated in the development of such diverse diseases as systemic lupus erythematosis, rheumatoid arthritis, immune glomerulonephritis, and vasculitis. We recently demonstrated that experimental immune complex–mediated injury in mice, as modeled by the cutaneous Arthus reaction, requires receptors for the Fc portion of the antibody and is unaffected by deficiencies in complement components. However, the responsible cell type(s) and Fc receptor(s) were not known. We now demonstrate by differential reconstitution in vivo that FcγRIII on mast cells is necessary for this inflammatory response. We propose a general model of antibody-mediated diseases as an immunopathologic spectrum whose specific manifestations are determined by the Fc receptor and cell type engaged.