What is a cytokine storm?

A cytokine storm occurs when the immune system loses its own brakes. Normally, a balance between attacking and dampening signalling molecules keeps the inflammatory response in check. When that balance is disrupted, cytokines are released in massive quantities -- proteins that activate other immune cells, which in turn produce even more cytokines. This self-reinforcing process can lead to damage to multiple organs simultaneously, or worse.

The first symptoms are vague: fever, fatigue, a general feeling of being unwell. That makes early intervention difficult. In a fully runaway cytokine storm, blood vessels leak fluid into the surrounding tissue, blood pressure drops dangerously, and the lungs become critically short of oxygen. Most people know the term from COVID-19, where severely ill patients had measurably elevated inflammatory markers while simultaneously showing reduced immune cell counts. This pattern was consistent with a cytokine storm, although the COVID-19 studies are relatively small and used varying definitions.

The cytokine storm is not a disease in itself but an end state that can occur with a wide range of underlying causes: severe infections, autoimmune diseases, and also as a side effect of modern cancer therapies. In CAR T-cell therapy, where genetically modified immune cells are injected to fight cancer, this occurs almost as a matter of course. It is then called cytokine release syndrome, or CRS. The severity varies: mild CRS manifests only as fever, while severe CRS requires intensive care.

When CRS becomes too severe, tocilizumab is the standard treatment. This drug blocks the action of a central signalling molecule (IL-6) in the cascade. It works, but the optimal moment of administration has not yet been precisely established. Doctors must also make a careful distinction between CRS and related syndromes, such as macrophage activation syndrome, because they resemble each other but sometimes require different treatments.