CD19 deficiency is a known genetic risk factor for the development of monogenic CVID in humans (1), and CD19−/− mice present with very similar humoral defects to those observed in human CVID patients (defective B cell development, terminal differentiation into isotype-switched plasma cells, and low serum titers of IgG, IgA and IgM) (18). Here, CD40LG is linked to common variable immunodeficiency.