IgA may be found in two forms. A monomeric form that is found in human serum, and a dimeric form s-IgA, which is found in secretions.
s-IgA is present in saliva, breast milk and gut secretions and with its resistance to digestive enzymes it is thought to provide surface protection at mucous membranes.
IgA is mainly produced by plasma cells in mucosa-associated lymphoid tissue (MALT) that is widely distributed throughout the Peyer's patches and lamina propria of the gut.
Unusually for immunoglobulin, IgA activates complement by the alternative pathway.
In brief (adults):
- High IgA (>4g/L)
- elevated IgA levels are nonspecific, but can be seen in pulmonary and gastrointestinal inflammatory diseases, some autoimmune conditions, liver disease, and plasma cell disorders
- Low IgA (<0.8g/L)
- partial or complete IgA deficiency affects up to 1% of the population, and is commonly asymptomatic. Patients may have a slightly higher risk of gastrointestinal diseases (including coeliac disease), autoimmune disease, or a modest increase in the rate of superficial infections
In detail:
Causes of increased IgA levels include:
- gamma-A myeloma (M component)
- chronic infections
- chronic liver disease
- rheumatoid arthritis with high titres of rheumatoid factor
- SLE (occurs in some patients)
- sarcoidosis (occurs in some patients)
- Wiskott-Aldrich syndrome
Causes of reduced IgA levels alone include:
- normal (occurs in approximately 1:700)
- herediatry telangiectasia
- malabsorption (some patients)
- SLE may also occasionally be associated with a reduced IgA
- liver cirrhosis (occasionally)
- Still's disease (occasionally)
- non-IgA myeloma
- Waldenstrom's macroglobulinaemia
- acquired deficiency
- type III dysgammaglobulinaemia
Causes of reduced IgA levels with other immunoglobulin decreases include:
- agammagloulinaemia e.g. congential or acquired (e.g. multiple myeloma, leukaemia, nephrotic syndrome, protein-losing enteropathy)
- heridatry thymic aplasia
- type I dysgammaglobulinaemia (decreased IgG and IgA and increased IgM)
- type II dysgammaglobulinaemia (absent IgA and IgM and normal levels of IgG)
- infancy, early childhood
Reference: