An accurate extended follow-up (clinical and immunological) in Immunology Clinics, even in asymptomatic patients, is crucial in order to evaluate the evolution of SIgMD in children and adults (persistent or transitory) and identify the possible progression to CVID or other well-defined immunodeficiencies, with a specific focus on patients who showed immune abnormalities in addition to low IgM [20,94]. Here, CD40LG is linked to selective IgM deficiency.