CD40LG and encephalitis: The detection of anti-JEV IgM in CSF is thought to be more specific for the diagnosis of acute JEV encephalitis than detection in sera because cross-reactive flavivirus antibodies are probably less likely to be found in CSF than sera and anti-JEV antibodies may persist longer in serum than in CSF.19 Serum-based IgM assays may also detect IgM antibodies resulting from JEV infections without encephalitis and persistence after JEV infection or vaccination.