Q.42 Which of the following statements about IgA is/are correct?
(A) It is secreted into colostrum.
(B) It is transported across the cell by transcytosis.
(C) Its secretion is facilitated by poly-Ig receptor.
(D) It primarily exists as a dimer in serum.
Correct answer: A, B, and C.
IgA, a key immunoglobulin in mucosal immunity, is accurately described by options A, B, and C, while D is incorrect due to its predominant monomeric form in serum.
Option Analysis
(A) Secreted into Colostrum
Colostrum, the first milk produced post-partum, contains high levels of secretory IgA (sIgA) for neonatal gut protection. Studies confirm IgA concentrations up to 5-9 g/L in human colostrum, decreasing in mature milk. This option is correct.
(B) Transported Across the Cell by Transcytosis
Dimeric IgA binds polymeric immunoglobulin receptor (pIgR) on the basolateral epithelial surface and undergoes transcytosis to the apical side for secretion into mucosa. This vesicular transport mechanism delivers sIgA to sites like the gut and lungs. This option is correct.
(C) Secretion Facilitated by Poly-Ig Receptor
The poly-Ig receptor (pIgR) specifically binds J-chain-linked dimeric IgA, enabling its transcytosis and cleavage to form secretory IgA with the secretory component. pIgR deficiency impairs mucosal IgA secretion. This option is correct.
(D) Primarily Exists as a Dimer in Serum
Serum IgA is mainly monomeric (about 83-90%), comprising 5-15% of total serum Ig, with only a minor dimeric fraction. Dimers predominate in secretions, not serum. This option is incorrect.
IgA, or Immunoglobulin A, plays a vital role in mucosal immunity, especially through its presence in colostrum, transport via transcytosis, and facilitation by the poly-Ig receptor. This article decodes key statements about IgA for CSIR NET aspirants, clarifying its forms like serum dimer misconceptions.
IgA Secretion in Colostrum
IgA dominates colostrum as secretory IgA (sIgA), providing passive immunity to newborns against gut pathogens. Concentrations range from 2.6-9 g/L, far higher than in mature milk.
Transcytosis Mechanism
Epithelial cells use transcytosis to move dimeric IgA from basolateral to apical surfaces, ensuring mucosal delivery. This process involves endosomal sorting via Rab11.
Poly-Ig Receptor Function
The poly-Ig receptor (pIgR) binds dimeric IgA via its J-chain, driving transcytosis and adding the secretory component post-cleavage. It handles both IgA and IgM polymers.
Serum IgA Form
Unlike secretory sites, serum IgA exists primarily as monomers (IgA1 subclass), not dimers, making “primarily dimer” inaccurate. Dimers are minor (~10-17%).
