Immunoglobulin A (IgA)

Summary

Immunoglobulin A (IgA) is a major class of antibodies found in secretions like tears, saliva, mucus, and notably, colostrum or breast milk. It is secreted in a dimeric form, as two antibodies connected by a joining chain. IgA provides immunity at mucosal membranes like the GI and respiratory tracts, and is importantly made by B-cells located in Peyer’s patches of the intestines.

Key Points

• Immunoglobulin A (IgA)
  • Released into secretions (tears, saliva, mucus, and breast milk)
    • colostrum (early breast milk) is high in IgA
    • most produced antibody overall, but serum concentration is low
  • Secreted as dimer connected by J-chain
    • picks up secretory component from epithelial cells, protecting the Fc portion from luminal proteases
  • Provides immunity at mucous membranes (mucosal immunity)
    • produced in GI tract (e.g. Peyer’s patches) and in airways
      • Prevents mucosal infections (esp. Giardia)
    • Selective IgA Deficiency causes recurrent GI and respiratory infections
  • Usually does not fix complement
    • Circulating levels are very low since most IgA is secreted