Lymphopenia (670/μL; RV: 1200-4100*).Low switched memory B cells (0.97%; RV: 3-46*).Low IgG (363 mg/dL; RV: 639-1349**), IgA (6 mg/dL; RV: 70-312**) and IgM (50 mg/dL; RV: 56-352**).Normal proliferation assays.Recurrent respiratory infections, Giardia and H. pylori infections, bronchiectasis, psoriasis, acute promyelocytic leukemia. Here, CD40LG is linked to lymphopenia.