Eculizumab is a medication used to treat certain rare autoimmune diseases by inhibiting the complement system, which is part of the immune response. It specifically targets the protein C5, preventing it from being activated and thereby reducing inflammation and damage to tissues. This drug is primarily prescribed for conditions like paroxysmal nocturnal hemoglobinuria (PNH) and aHUS (atypical hemolytic uremic syndrome). Eculizumab is administered through intravenous infusion and requires careful monitoring due to potential side effects, including an increased risk of infections.