AQP4 and myeloid sarcoma: In AQP4 NMO, where a relapse is more likely to result in a permanent deficit than in MS and disease activity is more likely to return with B cell repopulation—in particular memory B cell repopulation [101, 124]—close monitoring of CD19 + and CD19 + CD27 + B cell depletion and reconstitution is used as a tool for relapse prevention and timely treatment, further improving the treatment benefit-risk ratio [125].