As a byproduct of B cell dysfunction, laboratory hallmarks of CVID include low IgG with concomitant low IgM and/or IgA, poor antibody responses following vaccination, markedly decreased isotype switched memory B cells (CD27+IgD-IgM+), and expansion of anergic B cells (CD21Low) (1). Here, CD40LG is linked to common variable immunodeficiency.