Immunoglobulin Image shown below is of-
Ans: A. IgA
- IgA is the major immunoglobulin responsible for mucosal immunity.
- The levels of IgA in the serum are low, consisting of only 10–15% of total serum immunoglobulins present. In contrast, IgA is the predominant class of immunoglobulin found in extravascular secretions.
- In serum, IgA is secreted as a monomer resembling IgG. In mucous secretions, IgA is a dimer and is referred to as secretory IgA.
- This secretory IgA consists of two monomers that contain two additional polypeptides: the J chain that stabilizes the molecule and a secretory component that is incorporated into the secretory IgA when it is transported through an epithelial cell
- Immunoglobulin A (IgA) is present in the serum as a 170 kDa, four polypeptide (two L and two H) chain protein.
- Its H-chain type is alpha (α).
- IgA exists in serum in both monomeric and dimeric forms.
- Although it exists primarily in monomeric form, followed by dimeric, trimeric and some tetrameric forms are also present.
- IgA in blood occurs in monomeric form whereas those in body secretion occur in dimeric or multimeric forms.
- In secretions, in addition to the κ or λ L-chains and the IgA heavy chain alpha, IgA also contains two other polypeptide chains – secretory component (SC) and J-chain (Joining chain).
- Secretory chains help in transcytosis of exocrine IgA and stabilize IgA against proteolytic degradation.
- The two four-chain units are held together by the J-chain through disulfide bridges.