Rebecca Martin/Sandbox1

The most extensive surface in contact with the external environment is not our skin, but the epithelial lining of our gastrointestinal, respiratory, and urogenital tracts <ref name="seven">PMID:17428798</ref>. As a first line of defense in maintainance the integrity our mucosa, the immune system manufatures and secretes dimeric IgA to neutralize pathogenic organisms <ref name="five">PMID:15111057</ref> and exclude the entry of commensals at the mucosal border <ref name="nineseven">PMID:19079336</ref>. In the serum, IgA functions as a second line of defense against pathogens that may breech the epithelial boundary <ref name="five" /ref>. The body produces more IgA than any other antibody isotype <ref name="nineseven" /ref>. In fact, IgA is the most abundant antibody in the body, further illustrating IgA's critical role in immunity <ref name="ten">PMID:10064707</ref>. Exploring IgA's structure and protein interactions illuminates the unique and critical function IgA plays in humoral immunity.

Unlike other antibody isotypes, IgA exists in mutiple oligomeric states, the a monomeric, a dimeric, and secretory forms being most common <ref name="ten" /ref>,<ref name="nineseven" /ref>. At least two isotypes exist, termed IgA1 and IgA2. IgA2 can further be categorized into 2 allotypes: IgA2 m(1) and IgA2 m(2). The receptors for IgA include the Fcα Receptor (FcαRI; CD89) and the polyimmunologlobulin receptor (pIgRI). When binding to FcαRI results in the dimerization, the consequent signaling results in effector functions, including respiratory burst, phaocytosis, and eosinophil degranulation. Binding to the pIgR results in transoocytosis and IgA secretion <ref name="five" /ref>. At the mucosal surface, an approximately equal ratio of secretory IgA1 (sIgA1) to secretory IgA2 (sIgA2) reside at the mucosal surface, with the exception of the colon, where the majority is sIgA2 <ref name="nineten" /ref>. In the serum, about 90% of the IgA is monomeric IgA1 <ref name ="ten" /ref>.