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Immunoglobulin A (IgA) is one of the antibody molecules produced by the body to defend itself against foreign microorganisms. This antibody is part of the secretory immune system and is produced by cells in the mucus membranes of the respiratory system and the intestines. IgA protects the body by binding to foreign pathogens and blocking their entry into the body. Secretory IgA is the primary antibody found in secretions such as saliva, tears, mucus, and breast milk.
The IgA antibody has the same basic protein structure of other immunoglobulins. Two heavy polypeptide chains are bound to each other by disulfide bonds. Linked to the outer half of each heavy chain is a shorter light polypeptide chain. The immunoglobulin A molecule has an Fc region at the base that contains only the heavy chains. On the opposite ends of the IgA molecule, the region with both heavy and light chains is referred to as the Fab or antibody binding region.
When the IgA molecule is circulating in the blood, it exists as a single molecule known as a monomer. Two IgA molecules that are joined together are referred to as a dimer. The dimeric form of immunoglobulin A is found in the mucus membranes of the gut and the respiratory tract. Plasma cells in these membranes secrete a single IgA molecule that is attached to a polypeptide chain known as the J chain. This J chain functions to join two IgA molecules together to form a dimer.
After the dimer is formed, it binds to a receptor on the surface of gut epithelial cells known as secretory component. The resulting molecule is then referred to as secretory IgA and it is pumped into the lumen — the inner space — of the gut or lungs. Once in the lumen, the secretory IgA can bind to bacteria and prevent them from entering the body.
Since most bacteria enter the body through the mucus membranes, secretory IgA plays a very important role in the immune system. The amount of immunoglobulin A produced by the plasma cells in the gut is greater than that of any of the other immunoglobulins. Two subclasses of immunoglobulin A have been identified. These are IgA1 and IgA2 with IgA2 being the predominant form found in secretions. IgA2 is more resistant to breakdown by enzymes found in the gut.
Patients who are deficient in IgA commonly lack the plasma cells that produce this antibody. These patients may have no symptoms or they could experience frequent bouts of respiratory or gastrointestinal infections. Treatment usually centers on treating these infections with antibiotics and educating the patients about this deficiency.