Selective IgA deficiency is the most common form of immunodeficiency. Certain select populations, including allergic individuals, patients with autoimmune and gastrointestinal tract disease and patients with recurrent upper respiratory tract illnesses, have an increased incidence of this disorder. These patients have the unique ability to form various antibodies and auto-antibodies including anti-IgA antibodies. Failure of terminal differentiation of the B lymphocyte is the primary defect in IgA deficiency. Treatment should be directed toward the underlying disease associated with this immune disorder. These patients should be advised of the consequences of improperly administered blood or blood products.