Eculizumab injection is used to treat paroxysmal nocturnal hemoglobinuria (PNH: a type of anemia in which too many red blood cells are broken down in the body, so there are not enough healthy cells to bring oxygen to all parts of the body). Eculizumab injection is also used to treat atypical hemolytic uremic syndrome (aHUS; an inherited condition in which small blood clots form in the body and may cause damage to the blood vessels, blood cells, kidneys, and other parts of the body). Eculizumab injection is also used to treat a certain form of myasthenia gravis (MG; a disorder of the nervous system that causes muscle weakness). It is also used to treat neuromyelitis optica spectrum disorder (NMOSD; an autoimmune disorder of the nervous system that affects eye nerves and the spinal cord) in certain adults. Eculizumab injection is in a group of medications called monoclonal antibodies. It works by blocking the activity of the part of the immune system that may damage blood cells in people with PNH and that causes clots to form in people with aHUS. It also works by blocking the activity of the part of the immune system that may damage certain parts of the central nervous system in people with NMOSD or by disrupting communication between nerves and muscles in people with MG.