Many other diseases (e.g., CTLA4- Cytotoxic T-Lymphocyte Antigen 4- deficiency, LRBA- LPS responsive beige-like anchor protein- deficiency, CD40/CD40L deficiency), prior to the discovery of their monogenic cause, were classified as CVID, which is a heterogeneous group (the term “variable” refers to this heterogeneity) of disorders characterized by predominant antibody deficiency not due to other well-defined PIDs. Here, CD40 is linked to agammaglobulinemia.