3.2 Protection of mucosal surfaces

It was mentioned earlier that IgA antibodies can be produced as secreted molecules. In this case, the antibody is transferred across epithelial cells from the basal (tissue) side to the apical (exterior) side of the epithelium exposed at the mucosal surface. You can see the mechanism outlined in Figure 11.

Figure 11 Transport of IgA

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The IgA dimer, with its linking J chain, emerges from the plasma cell and binds to the poly-Ig receptor on the submucosal tissue side of an epithelial cell. The bound IgA is endocytosed, transported across the cell in a vesicle and is released at the mucosal surface, by cleavage of the poly-Ig receptor. The segment of the receptor that remains bound to the Fc portions of the secreted IgA dimer is the secretory component, which helps prevent enzymic degradation of the antibody. The other segment remains in the membrane.

Figure 11 Transport of IgA

Dimeric IgA produced by plasma cells binds to a poly-Ig receptor on the basal surface of an epithelial cell. It is transported across the cell in a vesicle and then released at the mucosal surface as secreted IgA. The secretory component of secreted IgA is derived from the poly-Ig receptor. IgA in mucosal secretions is particularly important in protecting the epithelium against infection with respiratory viruses, such as influenza, rhinoviruses and SARS-CoV2.