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