Various studies have shown an association between low levels of both IgG and IgM and rituximab therapy.1, , , , , –7 Two recent studies evaluating the long-term effect of rituximab on Ig levels in patients with neuromyelitis optica spectrum disorders discovered reductions in both IgG and IgM over time.8,9 In some studies, a prolonged hypogammaglobulinemia even years after the last treatment with rituximab was demonstrated.5,10. This evidence concerns the gene CD40LG and neuromyelitis optica.