Selective IgA deficiency is the most common primary immunodeficiency disorder and is characterized by the absence of serum IgA with normal levels of IgG and IgM; its prevalence is about 1:500 individuals. Most patients are asymptomatic because of compensatory increases in secreted IgG and IgM. Some affected patients have frequent and recurrent infections such as sinusitis, otitis, and bronchitis. Some cases of IgA deficiency may spontaneously remit. When IgG₂ subclass deficiency occurs in combination with IgA deficiency, affected patients are more susceptible to encapsulated bacteria and the degree of immune impairment can be more severe. Patients with a combined IgA and IgG subclass deficiency should be assessed for functional antibody responses to glycoprotein antigen immunization.

Atopic disease and autoimmune disorders can be associated with IgA deficiency. Occasionally, a sprue-like syndrome with steatorrhea has been associated with an isolated IgA deficit. Treatment with commercial immune globulin is ineffective, since IgA and IgM are present only in trace quantities in these preparations. Frequent infusions of plasma (containing IgA) or unwashed blood transfusions are hazardous, since anti-IgA antibodies may develop, resulting in systemic anaphylaxis or serum sickness.

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_{Cunningham-Rundles C: Physiology of IgA and IgA deficiency. J Clin Immunol 2001;21:303.}