The most common toxicities were infusional fevers, developingwithin the initial 24 h and only lasting for up to few days, and two mechanistically distinct delayed responses: cytokine release syndrome (CRS), characterised by dramatic (>1000×) IL6 and IFNγ elevation and in some cases respiratory and haemodynamic instability, and macrophage activation syndrome (MAS) with very high levels of ferritin and coagulopathy. Here, IFNG is linked to macrophage activation syndrome.