Secretory component immunoglobulin

The polyvalency of IgM causes it to bind more firmly to an antigen than either a univalent or bivalent antibody. In this respect, a single molecule of IgM can cause lysis of a cell. Furthermore, the joining chain (J) has been detected in polymeric IgM and IgA, but not in the other immunoglobulins. About 10% of the IgM in external secretions has the secretory component attached. One wonders whether these characteristics of the IglNI are responsible for its take-over role in the gut, when there is a defective synthesis of IgA as in celiac or in Crohn s disease or in ulcerative colitis. 

Hammerschmidt, S., Talay, S. R., Brandtzaeg, P., and Chhatwal, G. S. (1997). SpsA, a novel pneumococcal surface protein with specific binding to secretory immunoglobulin A and secretory component. Mol. Microbiol. 25,1113-1124. 

Butler, J. E. and Maxwell, C. F. 1972. Preparation of bovine immunoglobulins and free secretory component and their specific antisera. J. Dairy Sci. 55, 151-164. 

IgA is the primary secretory immunoglobulin and occurs in tears, nasal and intestinal secretions, saliva, and bile. Il is present as a 7S IgA, it is an I IS dimer with the monomeric structures joined by a J chain and linked to a glycoprotein called secretory component. Secretary IgA plays a role in initial protection against external pathogens, aggregates bacteria, and neutralizes viruses. 

Gurevich P, Zusman I, Moldavsky M, et al. Secretory immune system in human intrauterine development immunopathomorphological analysis of the role of secretory component (plgR/SC) in immunoglobulin transport (review). Int J Mol Med 2003 12(3) 289-97. 

The poly-Ig receptor has been cloned and sequenced [89] to reveal that the poly-Ig binding portion, i.e. secretory component, is composed of five highly conserved domains of approximately 100 amino acids which show considerable homology with immunoglobulin domains. It is possible that this arrangement of secretory components in domains facilitates interaction with the constant domains of IgA. Secretory component also becomes disulphide-linked to one of the monomers of dimeric IgA [90], A recent model suggests that it is a cysteine on the first domain of secretory component which links to the unpaired cysteine of the Ca2 domain [77], Fig. 13(b) shows a schematic representation of dimeric secretory IgA. In contrast to serum IgA, secretory IgA shows roughly equal proportions of the two subclasses. 

The J (joining) chain (reviewed by Koshland, 1975 and 1985 [124,125]) is a polypeptide chain of a molecular weight of about 15000. Unlike heavy and light chains, which contribute to all immunoglobulin molecules, the J chain is only attached covalently via disulphide bridges to the Fc portion of secreted IgM and IgA [126,127]. J chain associated with IgM or IgA shows a high affinity for the secretory component protein [128] and it is therefore believed to be necessary for rapid transport through secretory epithelial cells into exocrine fluids [129],... 

Figure 4.29 Models of IgG, IgA, IgD, IgE and IgM. (a) Structural model of IgGj before and after fragmentation by pepsin and papain and reduction with a sulphydryl reagent. Solid black chain portion = variable regions light chain portion = constant regions. Small black lines represent disulphide and half-cystine (—SH) groups. Small black dots in Fc regions represent attached carbohydrate groups. The various parts of the model are labelled, (b) The structure of four classes of immunoglobulins are shown with monomeric IgA, dimeric IgA and secretory IgA. Location of the J-chain, secretory component (SC) and carbohydrate is approximate.

Besides L and H chains, secretory IgA contains a polypeptide called secretory component, or SC, which has a molecular weight of 65,000 — 70,000 and is bound to the H chains of the molecule. Another component, J chain (J for joining mol. wt. 15,000), is found in the IgM of many primitive and advanced species, and in polymeric, but not monomeric (four-chain) IgA. J chain is linked to H chains through disulfide bonds. The amino acid sequences and antigenic properties of SC and J chain do not appear to be homologous to those of the immunoglobulin polypeptides i.e., they do not seem to have a common evolutionary origin. 

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