Schematic view of complement neutrophil activation amplification loop. Neutrophils stimulated by suboptimal doses of cytokine or coagulation-derived factors, possibly via the release of C5a, (1) secrete properdin, which triggers the complement AP on the neutrophil membrane. (2) This further activates neutrophils and enhances cell responses, possibly via the juxtamembranous release of complement-derived active fragments. (3)