VMC 221: Veterinary Immunology and Serology (1+1)

IMMUNOGLOBULIN A (Ig A)

• It is produced by plasma cells present mainly in the intestinal tract, respiratory tract, urinary tract, mammary gland and skin.
• Serum concentration is usually lower than IgM (except in humans).
• It is the major Immunoglobulin found in colostrums, saliva and tears.
• IgA has sedimentation coefficient of 7S with molecular weight of 160 KDa to 360 KDa.

Structure

• IgA molecule has two α heavy chains containing 3 constant domains (C_H1, C_H2, C_H3) and either two κ or two λ light chains.
• IgA occurs in two forms.
  • Serum IgA
  • Secretary IgA
• Serum IgA is a monomer (7S with molecular weight 160 KDa).
• But secretary IgA (SIgA) usually found in mucosal surfaces and in secretions is a dimer.
• It is formed by two 7S IgA monomer joined at their carboxy terminus of Fc region by “J” chain and also with a secretary component (Sc).
• “J” chain is synthesized by the same plasma cells.
• Secretary component is a glycoprotein (71 KDa), synthesized in the epithelial cells of the mucous membrane (not by plasma cells) and present as coil about the double F_C cylinders.
• Synthesis of Sc is independent of the production of IgA.
• The dimer IgA (SIgA) is much longer (11S with molecular weight of 360KDa).
• The Sc protects the SIgA from proteolysis by the gastrointestinal enzymes.
• Two IgA sub classes have been identified IgA1 and IgA2.
• The sub class IgA2 lacks disulphide bonds between the heavy and light chains. It is a minor component in serum but more present in secretions.
• The sub classes IgA1 and IgA2 were recorded in humans, cat, mouse, sheep and pigs. IgA posses about 7% carbohydrate.

Functions

• Provide local immunity to mucosal surface of respiratory and intestinal tract.
• It does not fix complement but can activate alternative complement pathway.
• It helps in phagocytosis and intracellular killing of microorganisms
• It is a minor component in systemic humoral immunity but plays a major role in mucosal immunity. IgA antibodies found in gut contents or feces are known as copro antibodies.