Cytokine Storm

https://medical-dictionary.thefreedictionary.com/cytokine+storm

A potentially fatal hyperrelease of inflammatory mediators in response to stimulation of T cells and macrophages by pathogens and immune insults Triggers Graft versus host disease, adult respiratory distress syndrome, sepsis, avian influenza, and systemic inflammatory response syndrome 

Cytokine Storm 

The massive release of interleukins, tumor necrosis factor alpha, and other circulating mediators of inflammation during critical illness. 

These agents may trigger bleeding, clotting, internal organ injury, or shock.

Broadly speaking, cytokine storm is a cascade of exaggerated immune responses that can cause serious problems.  The immune system contains many different components that help you battle infections.  It includes many different types of cells that communicate with each other via signaling molecules, known as cytokines.