What is a cytokines storm?

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. The name 'cytokine' is derived from the Greek words for cell (cyto) and movement (kinos). Sometimes the body's response to infection can go into overdrive. For example, when SARS - CoV-2 - the virus behind the covid-19 pandemic - enters the lungs, it triggers an immune response, attracting immune cells to the region to attack the virus, resulting in localised inflammation. But in some patients, excessive or uncontrolled levels of cytokines are released which then activate more immune cells, resulting in hyperinflammation. This can seriously harm or even kill the patient. Cytokine storms are a common complication not only of covid-19 and flu but of other respiratory diseases caused by coronaviruses such as SARS and MERS. They are also associated with non-infectious diseases such as multiple sclerosis and pancreatitis.

A cytokine storm is an overproduction of immune cells and their activating compounds (cytokines), which, in a flu infection, is often associated with a surge of activated immune cells into the lungs.