CD79A and isolated agammaglobulinemia: PADs include Common Variable Immunodeficiency (CVID), which is very rare and characterized by low IgG and IgA levels; X-linked agammaglobulinemia (XLA), also very rare and marked by a near-complete absence of B cells with very low levels of all Ig classes; selective IgA deficiency, whose frequency is 1:150 to 1:2000 in Caucasians but very rare in the Japanese; IgG subclass deficiency, whose prevalence can be up to 2% of healthy individuals when considering only low levels; other less common defects.