Neutralization by Polymeric Immunoglobulin A
Polymeric IgA is synthesized by plasma cells of the mucosal immune system which comprises linked elements of the gut and respiratory systems and protects mucosal surfaces (Brandtzaeg 1989; McGhee and Mestecky 1990). It gets there by binding to a poly-Ig receptor on the basal surface of an epithelial cell and being internalized in an endocytic vesicle which is transported to the apical surface membrane with which it fuses. It is then released by proteolytic cleavage of the poly-Ig receptor, part of which becomes the’ secretory component’ of the IgA molecule (Underdown 1990). IgA is a dimer in man but this and higher multiples are found in mice. Monomers and polymers are also found in serum, and these lack the secretory component. More IgA is synthesized than any other immunoglobulin isotype but as it is turned over more quickly there is less IgA than IgG. The structure of IgA has been described by Feinstein et al. (1971). Dimensions are listed in Table 5.