The monomeric form of IgA accounts for about 10% to 13% of antibodies in the blood of healthy individuals. Most IgA antibodies, however, are the secreted form, called secretory IgA (sIgA). In fact, IgA is the most abundant immunoglobulin class produced, even though it makes up only a small fraction of the antibodies in blood. The secreted form is important in mucosal immunity and is found on the mucous membranes that line the gastrointestinal, genitourinary, and respiratory tracts and in secretions such as saliva, tears, and breast milk. Secretory IgA in breast milk protects infants that are breast-fed against intestinal pathogens; it is not absorbed into the bloodstream of the child, but it is important in defending the gastrointestinal tract of the infant from infection and diarrheal diseases. ■ mucosal immunity, p. 397

Protection by secretory IgA is primarily due to the direct effect that binding of antibody has on antigens, including neutralization of toxins and viruses and interference with the attachment of microorganisms to host cells. IgA only weakly facilitates phagocytosis and complement activation, which reflects the limited involvement that these innate defenses typically have in the protection of mucosal membranes.

Production of secretory IgA occurs primarily in plasma cells that reside in the mucosal-associated lymphoid tissues (MALT). Recall that plasma cells are the antibody-secreting form of B cells. The sIgA molecule is a dimer, composed of two monomeric antibody subunits connected by a J-chain (J stands for joining). The dimer binds to the basement membrane side of the epithelial cells of the mucosa and is then transported through those cells to the surface of the mucosal lining. As part of the transport process, a polypeptide called the secretory component is added to the molecule; this component may help protect the antibody from being destroyed by most proteolytic enzymes that might be encountered in the mucosa. ■ MALT, p. 397

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