Clinical Significance

Approximately 10-15% of serum immunoglobulin is IgA.  Serum IgA is predominantly found in monomeric form with 10-15% as a dimer.  Secretory IgA, found in tears, sweat, saliva, milk, colostrum and gastrointestinal and bronchial secretions, is synthesized mainly by plasma cells in mucous membranes and breast ductules.  IgA can initiate complement activation by alternative pathway.  Secretory IgA plays a major role in the protection of the respiratory, genitourinary, and gastrointestinal tracts against infection.  The specific role of serum IgA is unclear, but may be important in virus neutralization.  Indications for serum IgA quantification include recurrent infections, anaphylactic transfusion reactions, differentiation of M-components in myeloma, and evaluation of progression of IgA myeloma.
IgA does not cross the placenta, and as a result, IgA levels in infants’ sera are very low.  Serum IgA levels do not reach adult concentrations until 12 years of age. Approximately one in every 700 caucasians is genetically IgA deficient.  Individuals with absent IgA have a higher than expected incidence of rheumatic disorders and lymphoma.  Secondary IgA deficiency is seen in non-IgA or multiple myeloma or macroglobulinemia, and with nephrotic syndrome.
Elevated IgA levels are associated with both polyclonal as well as monoclonal increases.  Polyclonal increases include chronic liver disease, chronic infections, and neoplasia of the lower GI tract, inflammatory bowel disease, and autoimmune disease such as rheumatoid arthritis. Monoclonal increases include IgA multiple myeloma and occasionally other lymphomas.