The role of IgA in local immune protection

The secretions that bathe the mucosal surfaces contain an array of host defense factors that differ in composition and concentration from those in the serum. Most of the immunoglobulins in external secretions exist as polymers of the basic four-chain Ig units; IgA is the major isotype. The unique cellular pathway that results in the production of IgA by plasma cells at the local mucosal level is linked by proximity to an epithelial mechanism that selectively transports polymeric IgA (as well as IgM) into the secretions and converts them into secretory immunoglobulins (SIgA and SIgM). Because of their unique structure, these secretory immunoglobulins are well adapted to function, along with the other secretory immune factors, in the external secretions. The rate of SIgA production may also be regulated at the level of the epithelial cell.