Secretory component

Figure 50-10. Schematic representation of serum igA, secretory igA, and igM. Both igA and igM have a J chain, but oniy secretory igA has a secretory component. Poiypeptide chains are represented by thick iines disuifide bonds iinking different poiypeptide chains are represented by thin iines. (Reproduced, with permission, from ParslowTG et al [editors] Medical Immunology, 10th ed. McGraw-Hill, 2001.)...

This occurs in the seromucous secretions such as saliva, tears, nasal secretions, sweat, colostrum and secretions of the lung, urinogenital and gastrointestinal tracts. Its purpose appears to be to protect the external surfaces of the body from microbial attack. It occurs as a dimer in these secretions but as a monomer in human plasma, where its function is not known. The function of IgA appears to be to prevent the adherence of microorganisms to the surface ofmucosal cells thus preventing them entering the body tissues. It is protected from proteolysis by combination with another protein—the secretory component. 

Brandtzaeg P, Rognum TO. Evaluation of nine different fixatives. 2. Preservation of IgG, IgA and secretory component in an artificial immunohistochemical test substrate. Histochemistry 1984 81 213-219. 

Vantrappen G, Peeters TL, Janssens J The secretory component of the interdigestive migrating motor complex in man. Scand J Gastroenterol 1979 14 663-667. 

The ability to stack genes in transgenic plants by successive crosses between individually transformed parental plants is a considerable advantage in attempting to construct multimeric protein complexes, such as secretory antibodies. As described earher, slgA consists of two basic Ig monomeric units (heavy and tight chains) that are dimerized by a joining (J) chain and then associated with a fourth polypeptide, the secretory component (SC) [36]. 

Dimer in external secretion caries secretory component. 

Dimers and polymers of IgA are found in the various external secretions such as gastrointestinal, colostrum, seminal, and vaginal secretions, saliva, tears, nasal, bile, urine, and tracheobronchial secretions. Parotid saliva has an IgGrIgA ratio of about 0.01 compared to 4.5 for serum (T5). Four types of polypeptide chains have been identified in S IgA the light chain, MW 22,500 the hea y chain, MW 55,000 the secretory component (SC), MW 60,000 and the joining (J) chain, MW 20,000. 

The joining chain (J in Figs. 1 and 2) is attached to IgA in the submucosa, forming the 10 S dimer which, by the process of pinocytosis, enters the mucosa and there becomes attached to the secretory component (SC), giving rise to the 11 S dimer. Secretory component prevents hydrolysis of S IgA in the lumen of the gut. The 7 S IgA monomer does not become attached to the J chain but enters the circulation via the venous-lymphatic circulation. This physicochemical property of IgA probably accounts for its relatively increased values in the serum of some children with kwashiorkor. 

Fig. 2. Diagram representing the comparative stmctures of secretory IgA and IgM. L, light chain H, heavy chain J, joining chain SC, secretory component. Reproduced by courtesy of the New England Journal of Medicine.

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. 

Dallas, S. D., and Rolfe, R. D. (1998). Binding of Clostridium difficile toxin A to human milk secretory component. /. Med. Microbiol. 47, 879-888. 

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. 

The major antibody isotype in external secretions is sIgA, and the total amount of IgA synthesized is twice the amount of IgG produced daily in humans. IgA cells represent up to 80% of the entire mucosal lymphoid cell population. sIgA in mucosal secretions results from polymeric IgA transported across mucosal epithelium via binding to the pIgRreceptor (also known as the secretory component). The receptor is eventually cleaved and results in an IgAipIgR complex, referred to as sIgA (Figure 7.2) (Rojas and Apodaca, 2002). 

Shimizu, N., Mori, N. and Kuwahara, Y. (1999). Identification of the new hydrocarbon (Z,Z)-l,6,9-heptadecatriene as the secretory component of Caloglyphuspolyphyllae (Astigmata Acaridae). Bioscience, Biotechnology and Biochemistry 63 ... 

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. 

Labib, S. R., Calvanico, N. J. and Tomasi, T. B. 1976. Bovine secretory component isolation, molecular size and shape, composition, and NH2-terminal amino acid sequence. J. Biol. Chem. 251, 1969-1974. 

Mach, J.-P. 1970. In vitro combination of human and bovine-free secretory component with IgA of various species. Nature 228, 1278-1282. 

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. 

Walker, M., Baz, A., Dematteis, S., Stettler, M., Gottstein, B., Schaller, J. and Hemphill, A. (2004) Isolation and characterization of a secretory component of Echinococcus multilocularis metacestodes potentially involved in modulating the host-parasite interface. Infection and Immunity 72, 527-536. 

Fig. 11.3. Simplified diagram showing structure of human secretory IgAl (slgAl). The secretory component which is probably wound around the IgA dimer, as shown, is now known to be a proteolytic fragment of a receptor (poly(Ig)receptor) see also Fig. 11.4. The J chain is required for joining the two subunits. (Modified from Roitt et at., 1986.)...

Uchida, T., Sato, K., Kawasaki, Y., and Dosako, S. 1996. Separation of lactoperoxidase, secretory component and lactoferrin from milk or whey with a cation exchange resin. US Patent No. 5,516,675. 

The polymeric IgA receptor/secretory component (plgA-R/SC, also designated plgR) mediated transport represents another, less common and less utilized, form of specialized endosomal epithelial transcytosis. plgR are found... 

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