According to the criteria of the European Society for Immunodeficiencies (ESID), the defining immunological features of CVID include hypogammaglobulinemia (reduced IgG and IgA, with or without decreased IgM), qualitative antibody deficiency (impaired vaccine response), as well as functional and phenotypic abnormalities in B-cell subsets, in particular a severe reduction of class-switched (CS) memory B-cells (MBCs) and plasmablasts (PBs), which represent the main source of antibody-secreting cells (ASCs) [2,3]. The gene discussed is CD40LG; the disease is agammaglobulinemia.