A treatment-free interval of 6 months at the age of 28 revealed mild hypogammaglobulinemia (serum IgG, mg/dl [normal range]: 697 [790–1700]) IgG1-subclass deficiency (serum IgG1, mg/dl [normal range]: 303 [500–880], normal levels of IgA, IgM, IgG2-4 and low pneumococcal IgG-antibodies despite the fact that recurrent pneumococcal infections had led to very high IgM-antibody titers (Fig 1A). Here, CD40LG is linked to agammaglobulinemia.