Immunoglobulin A (IgA)- Structure, Subclasses and Functions

  • Antibodies, or ‘immunoglobulins’, are glycoproteins that bind antigens with high specificity and affinity.
  • In humans there are five chemically and physically distinct classes of antibodies (IgG, IgA, IgM, IgD, IgE).
  • Immunoglobulin A (IgA), is the major class of antibody present in the mucosal secretions of most mammals.
  • It is the key first line of defence against invasion by inhaled and ingested pathogens at mucosal surfaces.

Immunoglobulin A (IgA)- Structure, Subclasses and Functions

  • It is present in mucous, tears, saliva, sweat, colostrum and secretions from the genitourinary tract, gastrointestinal tract, prostate and respiratory epithelium. It is also found in small amounts in blood. 
  • It comprises approximately 15% of the total serum immunoglobulin and thus is the second most common human immunoglobulin in serum with serum concentration of 1 to 4 mg/mL.
  • In humans it is encoded by two genes within the immunoglobulin gene locus on chromosome 14. The 5′ IgA gene encodes IgA1 while 3′ IgA gene encodes IgA2.

IgA subclasses

  • Two IgA subtypes exist in humans, IgA1 und IgA2.
  • They differ in the molecular mass of the heavy chains and in their concentration in serum.
  • IgA1 also differs from IgA2 in being susceptible to cleavage in its hinge region by proteases secreted by a number of different bacteria.
  • In addition to that, IgA2 occurs in two allotypic forms, IgA2 (m1) and IgA2 (m2).


– Predominant circulating IgA comprising approximately 85% of total IgA concentration in serum.

– IgA1 shows a good immune response to protein antigens and, to a lesser degree, polysaccharides and lipopolysaccharides.


– IgA2, represents only up to 15% of total IgA in serum, but IgA2 percentages are higher in secretions.

– Plays a crucial role in the mucosa of the airways, eyes and the gastrointestinal tract to fight against polysaccharide and lipopolysaccaride antigens.

Structure of IgA

  • Immunoglobulin A (IgA) is present in the serum as a 170 kDa, four polypeptide (two L and two H) chain protein.
  • Its H-chain type is alpha (α).
  • IgA exists in serum in both monomeric and dimeric forms.
  • Although it exists primarily in monomeric form, followed by dimeric, trimeric and some tetrameric forms are also present.
  • IgA in blood occurs in monomeric form whereas those in body secretion occur in dimeric or multimeric forms.
  • In secretions, in addition to the κ or λ L-chains and the IgA heavy chain alpha, IgA also contains two other polypeptide chains – secretory component (SC) and J-chain (Joining chain).
  • Secretory chains help in transcytosis of exocrine IgA and stabilize IgA against proteolytic degradation.
  • The two four-chain units are held together by the J-chain through disulfide bridges.

Functions of IgA

  • It primarily protects mucous membranes as IgA can cross the epithelial layer and enter into body secretion providing local immunity.
  • Secretory IgA provides the primary defense mechanism against some local infections because of its abundance in mucosal secretions (e.g., saliva and tears).
  • It prevents the passage of foreign substances into the circulatory system.
  • In body secretion IgA neutralize viruses and prevent attachment on host surface.
  • IgA is resistant to digestion and although a poor activator, can activate the complement pathway when aggregated.
  • Secretory IgA can also inhibit inflammatory effects of other immunoglobulins.


  4. Lydyard, P.M., Whelan,A.,& Fanger,M.W. (2005).Immunology (2 ed.).London: BIOS Scientific Publishers.
  5. Playfair, J., & Chain, B. (2001). Immunology at a Glance. London: Blackwell Publishing

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