Cytokine Release Syndrome (CRS), or Cytokine Storm Syndrome (CSS) is a life-threatening inflammatory condition most commonly associated with immunotherapies, though also triggered by other factors (drugs, infection, etc.). In the case of COVID-19, this syndrome should not be used interchangeably with "cytokine storm (CS)" (they have a shared phenotype), and the distinctions are important to clarify.

• both CRS/CSS and CS are illustrated by sharp significant increases in the circulation of different pro-inflammatory cytokines
• CRS/CSS (when resulting from immunotherapy) has delayed symptomatology (days to weeks)
• immediate onset CRS/CSS = CS