Patients with X-linked or Autosomal Recessive Agammaglobulinemia lack mature circulating B lymphocytes and present a severe decrease in all Ig subtypes, whereas patients with Common Variable Immunodeficiency (CVID) have dysfunctional B lymphocytes and decreased circulating levels of IgG, IgA and possibly IgM. The gene discussed is CD40LG; the disease is common variable immunodeficiency.