Immunoglobulin A (IgA), Subclasses (1-2)

Two classes of IgA have been identified in humans: IgA₁, which accounts for 80% to 90% of total serum IgA, and IgA₂ which is the major subclass in secretions such as milk. The two subclasses appear to be regulated independently. Antigenic sites on the IgA subclasses are responsible for the anaphylactic transfusion reactions experienced by some patients totally deficient in either IgA₁ or IgA₂. After repeated transfusions, such patients may produce antibodies to these antigens. IgA subclasses are of further importance in that certain pathogenic microörganisms, including Haemophilus influenzae, are capable of enzymic cleavage of IgA₁, leading to partial inactivation of this subclass. Recurrent sinopulmonary infections may, therefore, be related to deficiency of IgA₂, which is resistant to these organisms. Grossly elevated levels of either subclass can occur in patients with IgA myeloma.