Among the Hungarian cohort, those who developed severe AKI had lower alternative pathway activity (p = 0.008), classical pathway activity (p = 0.001), C3 (p = 0.002), and C4 (p = 0.002), while C3a (p = 0.005) and C3a/C3 ratio (p < 0.001) were significantly elevated in these patients. This evidence concerns the gene C3 and acute kidney injury.