This is a case of paroxysmal nocturnal hemoglobinuria (PNH). PNH occurs when mutations of a gene called PIG-A occur in a bone marrow stem cell. Flow cytometry reveals lack of CD 55 and CD 59. PNH is the only hemolytic anemia caused by an acquired (rather than inherited) intrinsic defect in the cell membrane (deficiency of glycophosphatidylinositol leading to the absence of protective proteins on the membrane). Eculizumab specifically binds to the terminal complement component 5, or C5, which acts at a late stage in the complement cascade. By inhibiting the complement cascade at this point, the normal, disease-preventing functions of proximal complement system are largely preserved, while the propeies of C5 that promote inflammation and cell destruction are impeded