CVID are the most frequent symptomatic primary immunodeficiencies diagnosed in adulthood, and presents as a heterogeneous condition characterized by hypogammaglobulinemia (especially IgG, IgA and/or IgM), poor antibody response to vaccinations and recurrent upper and/or lower airway infections, as well as several other systemic manifestations, ranging from autoimmunity to lymphoproliferative disorders [49,50]. Here, CD40LG is linked to agammaglobulinemia.