ECULIZUMAB treats certain blood conditions that can cause low levels of red blood cells (anemia) and blood clots, such as atypical hemolytic uremic syndrome (aHUS) and paroxysmal nocturnal hemoglobinuria (PNH). It works by slowing down an overactive immune system, which reduces the breakdown of red blood cells. It also prevents blood cells (platelets) from forming a clot. It may also be used to treat a condition that causes muscles to easily weaken or fatigue (myasthenia gravis).