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.

This is thought to allow IgA1 to adapt more effectively to varying epitope spacings on multivalent antigens, while also presenting less resistance to bacterial proteases.

The oligomeric forms of IgA in the external (mucosal) secretions also contain a polypeptide of a much larger molecular mass (70 kDa) called the secretory component that is produced by epithelial cells.

This molecule originates from the poly-Ig receptor (130 kDa) that is responsible for the uptake and transcellular transport of J chain-containing polymeric (but not monomeric, which is devoid of J chain) IgA across the epithelial cells and into secretions such as tears, saliva, sweat and gut fluid.

[citation needed] In the blood, IgA interacts with an Fc receptor called FcαRI (or CD89), which is expressed on immune effector cells, to initiate inflammatory reactions.

[15] Ligation of FcαRI by IgA containing immune complexes causes antibody-dependent cell-mediated cytotoxicity (ADCC), degranulation of eosinophils and basophils, phagocytosis by monocytes, macrophages, and neutrophils, and triggering of respiratory burst activity by polymorphonuclear leukocytes.

[15] Polymeric IgA (mainly the secretory dimer) is produced by plasma cells in the lamina propria adjacent to mucosal surfaces.

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.

[19] Immune exclusion is a process of agglutinating polyvalent antigens or pathogens by crosslinking them with antibody, trapping them in the mucus layer, and/or clearing them peristaltically.

[19] Since sIgA is a poor opsonin and activator of complement, simply binding a pathogen isn't necessarily enough to contain it—specific epitopes may have to be bound to sterically hinder access to the epithelium.

Additionally, Blastocystis species have been shown to have several subtypes that generate cysteine and aspartic protease enzymes which degrade human IgA.

HSP occurs usually in small children and involves the skin and connective tissues, scrotum, joints, gastrointestinal tract and kidneys.

Schematic of immunoglobulin A dimer showing H-chain (blue), L-chain (red), J-chain (magenta) and secretory component (yellow).
Two views, one rotated 90 degrees with respect to the other, of the amino acid chains comprising secretory IgA1 . Colors are: H-chains (blue and light blue), L-chains (red and light red), J-chain (magenta) and the secretory component (yellow). Coordinates of each backbone carbon atom were derived PDB entry 3CHN . [ 1 ]
Two views, one rotated 90 degrees with respect to the other, of the amino acid chains comprising secretory IgA2 . Colors are: H-chains (blue and light blue), L-chains (red and light red), J-chain (magenta) and the secretory component (yellow). Coordinates of each backbone carbon atom were derived PDB entry 3cm9 . [ 2 ]