Researchers at Washington University School of Medicine in St. Louis are investigating whether a drug approved by the Food and Drug Administration (FDA) to treat rare diseases of an overactive immune system could help critically ill patients hospitalized with COVID-19.

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The drug blocks a specific protein of the immune system that doctors suspect contributes to dangerous responses triggered by coronavirus infection in the lungs, kidneys and blood vessels.

The drug, called ravulizumab, is a monoclonal antibody that inhibits what is called the complement system, which sits at the intersection of the body's immune responses to infection and the coagulation cascade that controls blood clotting. The drug is approved by the Food and Drug Administration (FDA) to treat two rare genetic diseases, atypical hemolytic uremic syndrome and paroxysmal nocturnal hemoglobinuria, both of which cause life-threatening blood clots in small blood vessels.