Serum IgM is the first antibody secreted after exposure to foreign antigens and is effective at engaging complement to pathogens.24 Patients with selective IgM deficiency are prone to develop serious infections and autoimmune conditions.24 Among patients with AAV, serum IgM hypogammaglobulinaemia after therapy with RTX is well described. This evidence concerns the gene CD40LG and hyperinsulinemic hypoglycemia, familial, 4.