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IgA represents about 15% to 20% of immunoglobulins in the blood, although it is primarily secreted across the mucosal tract into the stomach and intestines. This prevents microbes from binding to epithelial cells in the digestive and respiratory tracts. This immunoglobulin helps to fight against pathogens that contact the body surface, are ingested, or are inhaled. It does not activate complement, and opsonises only weakly. Its heavy chains are of the type α. It exists in two forms, IgA1 (90%) and IgA2 (10%) that differ in the structure. IgA1 is composed like other proteins, however in IgA2 the heavy and light chains are not linked with disulfide but with noncovalent bonds. IgA1 is found in serum and made by bone marrow B cells, however IgA2 is made by B cells located in the mucosae and has been found to secrete into, colostrum, maternal milk, tears and saliva. The IgA found in secretions have a special form. They are dimeric molecules, linked by two additional chains. One of these is the J chain (from join), which is a polypeptide of molecular mass 1,5 kD, rich with cysteine and structurally completely different from other immunoglobulin chains. This chain is formed in the antibody-secreting cells. The dimeric form of IgA in the outer secretions also has a polypeptide of the same molecular mass (1,5 kD) called the secretory chain and is produced by epithelial cells. It is also possible to find trimeric and even tetrameric IgA. Decreased or absent IgA, termed selective IgA deficiency, can be a clinically significant immunodeficiency.
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