Cytokines and inflammatory response localized in the pulmonary alveoli. This medical illustration shows on the upper side the normal immune response when a virus (in red and yellow) penetrates into the alveoli of the lungs. While the lower part of the illustration represents an abnormal reaction that triggers a cytokine storm. In the normal reaction, the cytokines (light green beads) produced by macrophages (in green) when the virus arrives in the alveolus stimulate the inflammatory response. They also stimulate the multiplication of immune cells. Thus T lymphocytes (in purple) and macrophages destroy viruses. Then the transfer of oxygen to the blood system is almost normal. Except in the case of an abnormal reaction, the cytokines stimulate the inflammatory response in an exaggerated way. As a result, too many immune cells are activated. Dead cells and collagen deposits cause fibrosis of the alveoli (lumps on the inner wall of the alveoli). In conclusion, gas exchange becomes difficult. And in the end, they cause breathing difficulty.