Medicine & USMLE

Immunoglobulin A (IgA)

  1. Immunoglobulin A (IgA)
  2. Immunoglobulin G (IgG)
  3. Immunoglobulin M (IgM)
  4. Immunoglobulin E (IgE)
  5. Immunoglobulin D (IgD)


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