Experimental studies using mouse mutants lacking FcμR demonstrated that a deficiency in IgM causes the targeted response of IgG antibodies against T-dependent and T-independent antigens, ultimately leading to the emergence of autoimmune conditions.7 The accumulation of CD21low B cells expressing high activation markers may also play a role in the initiation of an autoimmune response in patients with SIGMD. Here, CD40LG is linked to selective IgM deficiency.