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The secretory component is a component of
immunoglobulin A Immunoglobulin A (Ig A, also referred to as sIgA in its secretory form) is an antibody that plays a role in the immune function of mucous membranes. The amount of IgA produced in association with mucosal membranes is greater than all other ty ...
(IgA). Secretory component is a proteolytic cleavage product of the
polymeric immunoglobulin receptor Polymeric immunoglobulin receptor (pIgR) is a transmembrane protein that in humans is encoded by the ''PIGR'' gene. It is an Fc receptor which facilitates the transcytosis of the soluble polymeric isoforms of immunoglobulin A and immunoglobulin M ...
which remains associated with dimeric IgA in sero-mucus secretions. Polymeric IgA binds to the polymeric immunoglobulin receptor on the
basolateral The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...
surface of epithelial cells and is taken up into the cell via
transcytosis Transcytosis (also known as cytopempsis) is a type of transcellular transport in which various macromolecules are transported across the interior of a cell. Macromolecules are captured in vesicles on one side of the cell, drawn across the cell, and ...
. The receptor-IgA complex passes through the cellular compartments before being secreted on the luminal surface of the epithelial cells, still attached to the receptor.
Proteolysis Proteolysis is the breakdown of proteins into smaller polypeptides or amino acids. Uncatalysed, the hydrolysis of peptide bonds is extremely slow, taking hundreds of years. Proteolysis is typically catalysed by cellular enzymes called protease ...
of the receptor occurs and the dimeric IgA molecule, along with the secretory component, are free to diffuse throughout the lumen. Secretory components wrap around two IgA units joined by a
J chain A J chain is a protein component of the antibodies IgM and IgA. It is a 137 residue polypeptide, encoded by the ''IGJ'' gene. Structure The J Chain's molecular weight is approximately 15 kDa. It exhibits a standard immunoglobulin folding s ...
protein fragment, resulting in a >--< configuration, with each of the two antigen binding regions of the two constituent y-shaped antibodies exposed. One identified function of secretory components is to protect IgA antibodies from degradation by the gastric acids and enzymes of the digestive system. This property is especially important in the transfer of immune system components during
breastfeeding Breastfeeding, or nursing, is the process by which human breast milk is fed to a child. Breast milk may be from the breast, or may be expressed by hand or pumped and fed to the infant. The World Health Organization (WHO) recommends that br ...
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References

{{Reflist Antibodies