Complement anaphylatoxins are raised in MS CSF (Ingram et al., 2010; Håkansson et al., 2020) and CSF C5a levels and C5a inhibition in the CSF ameliorates blood-brain barrier disruption (Faustmann et al., 1995; Flierl et al., 2009). Here, C5 is linked to myeloid sarcoma.