CD40LG and viral encephalitis: When a person has encephalitis or meningitis, a high IgM of West Nile virus antibodies is undoubtedly indicative of infection; however, the IgM may last for many months to more than a year because humoral IgM antibodies do not penetrate the blood–brain barrier, an intrathecal West Nile virus-specific IgM strongly supports a central nervous system infection (Kemmerly, 2003).