When bodies fight back against infection, they can overwhelm themselves with their own destructive force.
In John M. Barry’s book on the 1918 flu pandemic, The Great Influenza, one of the most striking passages concerns what actually killed many of the young, healthy victims of the infection, causing them to turn blue and bleed from the nose or ears:
The virus was often so efficient at invading the lungs that the immune system had to mount a massive response to it. What was killing young adults a few days after the first symptom was not the virus. The killer was the massive immune response itself.
The phenomenon Barry describes is the cytokine storm, or cytokine release syndrome—when a body’s natural defenses are called into action and are so overwhelming and destructive that instead of rescuing the sick, they cause their death.
Cytokines are small signaling proteins, which can be grouped by their origins, structural characteristics, effects on the body, and signaling pathways. Each type has a corresponding receptor on the cell surface and, once received, can start one of a number of cellular tasks that begin a cascade of responses, depending on type.