Selective IgA deficiency (IgAD) is the most common primary immunodeficiency with a wide range of clinical symptoms including respiratory and gastrointestinal infections, autoimmunity, allergic conditions, and, in some cases, progression to the more severe antibody deficiency, common variable immunodeficiency (CVID). Genetics, intrinsic defects in lymphocytes, and impairment of the cytokine profile are all thought to contribute to IgAD pathogenesis, but the underlying cause of the diseases still remains unknown. This, together with a wide range of clinical manifestations in the patients and the association between IgAD and other immune disorders, presents a challenge for clinicians and researchers in both diagnosing and treating IgAD.