Selective IgA deficiency is the most common primary immunodeficiency, with a prevalence of approximately 1/600 in whites. Most subjects are asymptomatic but some may suffer from frequent respiratory and gastrointestinal infections. Patients who suffer from frequent infections usually have a defect in antibody responses toward polysaccharides, which is often associated with IgG2 deficiency. Genetic predisposition to develop IgA deficiency has been shown to be linked to at least one locus on 6p21. Some IgA-deficient patients are also prone to develop more severe immunodeficiency called common variable immunodeficiency, which is associated with decreased IgG and sometimes IgM production as well as partial T-cell defect. In a few cases, IgA deficiency may reveal a severe disease such as ataxia-telangiectasia. Selective IgA deficiency contraindicates immunoglobulin administration. Only the minority of IgA-deficient patients who develop severe or frequent infections in association with IgG2 deficiency or impaired antibody response are candidates for prophylactic intravenous immunoglobulin substitution. Immunoglobulin preparations containing particularly low amounts of IgA are required to avoid adverse effects related to anti-IgA alloantibodies.