CD22‐Fv fragment, fused to Pseudomonas endotoxin A (PE38), moxetumomab pasudotox, Lumoxiti, earlier approved in 2018 for specific cases of relapsed or refractory hairy cell leukaemia was found to cause AEs including hypertension; febrile neutropenia; haemolytic uraemic syndrome (HUS) and capillary leak syndrome (CLS), attributed to the superantigen PE38, which were indicated as FDA label boxed warnings. Here, CD22 is linked to capillary leak syndrome.