An eight year old child brought by her mother complaining that her child is getting frequent chest infections since she was small. Serological examination shows deficiency of ceain immunoglobulin. Which of the following is the commonest immunoglobulin deficiency seen?

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.