Antineutrophil cytoplasmic Abs (ANCA) can activate neutrophils in an FcγR-dependent manner, but the link between this ANCA-induced effect and mononuclear cell activation with the characteristic granuloma formation of Wegener's granulomatosis is unclear. Human α-defensins, small cationic antimicrobial peptides, are found in neutrophils and have chemotactic activity for T cells, dendritic cells, and monocytes. In this study, we quantitated the release of α-defensins (human neutrophil peptides 1-3) from human neutrophils after targeted FcγR cross-linking (XL). Homotypic XL of FcγRIIa, FcγRIIIb, or heterotypic XL of both receptors resulted in significant release of α-defensins, an effect also induced by both human polyclonal and murine monoclonal cytoplasmic staining ANCA (anti-proteinase 3). This release of α-defensins, as well as of other granule constituents (ANCA targets anti-proteinase 3 and myeloperoxidase and elastase), was significantly greater in donors homozygous for the NA1 allele of FcγRIIIb than in donors homozygous for NA2. Interestingly, the ANCA-induced release was completely inhibited by the IgG Fc-binding peptide TG19320, which blocks the IgG-Fc region from binding to FcγR. Based on their chemotactic properties, α-defensins and their release by ANCA may contribute to modulation of the acquired immune response and to granuloma formation. The greater activity of the FcγRIIIB-NA1 genotype may also explain the greater severity of disease and its flare-ups in patients with this allele.