A main feature of COVID-19 is lung inflammation and respiratory failure caused by an overexuberant immune response known as the cytokine storm. But why does the body produce such an excess of cytokine immune cells? New research from Talal Chatila, MD, of the Division of Immunology at Boston Children’s Hospital and colleagues provides some insights, particularly regarding the mechanisms involved in severe lung inflammation in COVID-19. The study, published in Immunity, also provides some therapeutic suggestions to quiet the runaway immune response.