C3 and systemic lupus erythematosus: At least three factors may have contributed to the increased responsiveness of SLE PMNs to MPs in the autologous setting: the PMNs of SLE patients, particularly those with low C3 levels, may have been primed in vivo; serum components (e.g. opsonizing autoantibodies or increased levels of anaphylatoxins or pro-inflammatory cytokines) may promote PMN activation; and the MPs per se may have increased capacity for PMN activation (e.g. due to tagging with complement fragments [19] or antibodies [23]).