A dysgammaglobulinemia characterized by low or undetectable serum levels of immunoglobulin class A (IgA). It is the most common primary antibody deficiency. It may be inherited or the reversible sequela of infection or certain drugs. It may be caused by decreased or inefficient class-switching from progenitor B cells without any corresponding decreases in the other isotypes. Though affected persons may be asymptomatic, low levels of IgA will reduce the immune system's ability to combat infection where IgA is normally secreted, at mucosal surfaces. Selective IgA deficiency is seen in greater proportion among patients with autoimmune disorders. The disease is Monarch Disease Ontology entry MONDO_0001341 (selective IgA deficiency disease). Also known as: IgA deficiency, SIgAD, gamma-A-globulin deficiency, immunoglobulin A deficiency, selective IgA immunodeficiency, selective immunoglobulin A deficiency.