How is immunoglobulin A (IgA) synthesized and distributed?

Immunoglobulin A (IgA) is an important antibody found in mucous secretions. It is produced by plasma cells located in mucosal tissues. The newly produced IgA molecules dimerize or pair up intracellularly with the J chain, a small cysteine - rich polypeptide. This dimeric IgA binds to a specific receptor in the membrane of mucosal epithelial cells. 

Cells actively take in the IgA-receptor complex through endocytosis where it is first transported across the cytoplasm, and then released into external body fluids after cleaving the pIgR peptide chain. During this process, a part of the receptor remains attached to IgA, forming the secretory component. The whole molecule that goes through this process is known as secretory IgA.

Secretory IgA initially functions as a receptor on the cell membrane for the J chain-containing polymeric immunoglobulins. During transport across the epithelial cells, IgA binds to the secretory component through disulfide bonds. The secretory component is then cleaved from its membrane anchor before the complete secretory IgA is released. Plasma cells producing IgA migrate to subepithelial tissues. In these tissues, IgA binds to receptors for polymeric immunoglobulins.

Humans produce more secretory IgA daily than any other type of antibody, secreting between 5 to 15 grams into mucous secretions.

IgA synthesis is regulated by T cells, with a specific cytokine, TGF-β, prompting B cells to switch from producing IgM to producing IgA.