C5 and paroxysmal nocturnal hemoglobinuria: Eculizumab, a humanized monoclonal antibody directed against complement component C5, was first approved as treatment for paroxysmal nocturnal hemoglobinuria and atypical hemolytic-uremic syndrome, because it reduced complement-mediated intravascular hemolysis and thrombotic microangiopathy.10,11 Subsequent studies showed eculizumab also improved symptoms in complement-mediated neurological diseases, such as neuromyelitis optica spectrum disorders and refractory generalized myasthenia gravis.9,12, –14 In aSAH, early brain injury and delayed cerebral ischemia may also be complement-mediated.