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Selective immunoglobulin A (IgA) deficiency (SIgAD [1]) is a kind of immunodeficiency, a type of hypogammaglobulinemia. People with this deficiency lack immunoglobulin A (IgA), a type of antibody that protects against infections of the mucous membranes lining the mouth, airways, and digestive tract.
The IgA dimeric form is the most prevalent and, when it has bound the Secretory component, is also called secretory IgA (sIgA). sIgA is the main immunoglobulin found in mucous secretions , including tears , saliva , sweat , colostrum and secretions from the genitourinary tract , gastrointestinal tract , prostate and respiratory epithelium .
Secretory component is a proteolytic cleavage product of the polymeric immunoglobulin receptor which remains associated with dimeric IgA in sero-mucus secretions. [1] Polymeric IgA binds to the polymeric immunoglobulin receptor on the basolateral surface of epithelial cells and is taken up into the cell via transcytosis .
IgA antibodies are secreted in the respiratory or the intestinal tract and act as the main mediators of mucosal immunity. [13] They are monomeric in the serum, but appear as a dimer termed secretory IgA (sIgA) at mucosal surfaces. The secretory IgA is associated with a J-chain and another polypeptide chain called the secretory component. [14]
A basal protein of the pIgR known as secretory component (SC) recognizes Ig ready for secretion. [20] The binding between the secretory component and secretory Ig is facilitated by the antibody's J chain which makes physical contact with the secretory component in order to change the transporter's conformation to an open state. [21]
The MALT's cellular component is composed mostly of dendritic cells, macrophages, innate lymphoid cells, mucosal-associated invariant T cells, intraepithelial T cells, regulatory T cells (Treg), and IgA secreting plasma cells. [1] [3] [8] Intraepithelial T cells, usually CD8+, reside between mucosal epithelial cells.
Polymeric immunoglobulin receptor (pIgR) is a transmembrane protein that in humans is encoded by the PIGR gene. [5] It is an Fc receptor which facilitates the transcytosis of the soluble polymeric isoforms of immunoglobulin A and immunoglobulin M (pIg) and immune complexes. pIgRs are mainly located on the epithelial lining of mucosal surfaces of the gastrointestinal tract.
The IgG antibody is similar to AGA IgA, but is found at higher levels in patients with the IgA-less phenotype. It is also associated with coeliac disease and non-celiac gluten sensitivity. [5] [6] [7] Anti-gliadin antibodies are frequently found with anti-transglutaminase antibodies.