IgA deficiency is the most common of selective immunoglobulin deficiencies; IgG and IgM deficiencies are rarer. Patients with a deficiency of IgA typically have recurrent sinus and lung infections. The cause of IgA deficiency may be a failure of heavy chain gene switching because the amounts of IgG and IgM are normal. Patients with a deficiency of IgA should not be treated with gamma globulin preparations, because these patients may form antibodies against the foreign IgA and, by cross-reaction, deplete their already low level of IgA. Ref: Levinson W. (2012). Chapter 68. Immunodeficiency. In W. Levinson (Ed), Review of Medical Microbiology & Immunology, 12e.