Cytokine storm. Composite coloured scanning electron micrograph (SEM) of immune cells involved in a cytokine storm. A single macrophage is surrounded by numerous white blood cells. Diseases such as covid-19 and influenza can be fatal due to an overreaction of the body's immune system called a cytokine storm. Cytokines are small proteins released by many different cells in the body, including those of the immune system where they coordinate the body's response against infection and trigger inflammation. In the lungs cytokine activated macrophages and virally infected dendritic cells lead to a more extensive immune response and the initiation of a cytokine storm. Released cytokines attract more inflammatory cells to migrate from blood vessels into the site of inflammation, and these cells release additional cytokines to amplify the cytokine storm.