Selective IgA deficiency is defined by the European Society of Immunodeficiency as a serum IgA level < 7 mg/dL with normal levels of serum IgG and IgM in an individual older than 4 years in whom other causes of hypogammaglobulinemia have been excluded and who has a normal IgG antibody response to vaccination [70]. Here, CD79A is linked to selective IgA deficiency disease.