Coloured composite of scanning electron micrographs (SEMs) of immune cells illustrating a cytokine storm. The small cells are different types of leukocytes, the large cells are macrophages. Both are types of white blood cell. A cytokine storm is a severe immune reaction that results in greatly elevated levels of inflammatory immune proteins (cytokines, not seen) in the body. The high levels of inflammation cause damage and can be fatal, even in those that were otherwise fit and healthy. Cytokine storms can occur with viral or bacterial diseases, such as Covid-19, flu and sepsis, or with treatments such as CAR T cell immunotherapy. Magnification: x1,500 when printed at 15cm wide.