General Defense Mechanisms Associated with the Digestive System

Topics: Immune system, Hepatitis C, Bacteria Pages: 7 (1833 words) Published: March 11, 2006
Mary Gorton
Microbiology Research paper
Page 1

General defense mechanisms associated with the digestive system The gastrointestinal tract is a lymphoid organ, and the lymphoid tissue within it is collectively referred to as the gut-associated lymphoid tissue or GALT. The number of lymphocytes in the GALT is roughly equivalent to those in the spleen, and, based on location, these cells are distributed in three basic populations: 1.Peyer's Patches: These are lymphoid follicles similar in many ways to lymph nodes, located in the mucosa and extending into the submucosa of the small intestine, especially the ileum. In adults, B-lymphocytes predominate in Peyer's patches. Smaller lymphoid nodules can be found throughout the intestinal tract.

2.Lamina propria lymphocytes: These are lymphocytes scattered in the lamina propria of the mucosa. A majority of these cells are IgA-secreting B cells. 3.Intraepithelial lymphocytes: These are lymphcytes that are positioned in the basolateral spaces between lumenal epithelial cells, beneath the tight junctions (they are "inside" the epithelium, but not inside epithelial cells as the name may incorrectly suggest). Another important component of the GI immune system is the M or microfold cell. Mary Gorton

Microbiology Research paper
Page 2

M cells are a specific cell type in the intestinal epithelium over lymphoid follicles that endocytose a variety of protein and peptide antigens. Instead of digesting these proteins, M cells transport them into the underlying tissue, where they are taken up by local dendritic cells and macrophages. Dendritic cells and macrophages that receive antigens from M cells present them to T cells in the GALT, leading ultimately to appearance of immunoglobulin A-secreting plasma cells in the mucosa. Dendritic cells below the epithelium can also sample lumenal antigens by pushing pseudopods between epithelial cells. The secretory IgA is transported through the epithelial cells into the lumen, where, for example, it interferes with adhesion and invasion of bacteria. T cells exposed to antigen in Peyer's patches also migrate into the lamina propria and the epithelium, where they mature to cytotoxic T cells, providing another mechanism for containing microbial assaults. (Austgen PhD)

Typhoid fever
Salmonella typhi is a gram-negative, flagellate, nonencapsulated, nonsporulating, facultative anaerobic bacillus that ferments glucose, reduces nitrate to nitrite, and synthesizes peritrichous flagella when motile. S typhi has O and H antigens, an envelope (K) antigen, and a lipopolysaccharide macromolecular complex, called endotoxin, that forms the outer portion of the cell wall. (Tortora et al. 714-735) Mary Gorton

Microbiology Research paper
Page 3

Once S. Typhi bacteria are eaten or drank, they multiply and spread into the bloodstream. The body reacts with fever and other signs and symptoms. After an incubation period of 7-14 days, a systemic infection develops with symptoms including a prolonged fever, general malaise, and chills, swelling of the lymph nodes, rose-colored spots on the chest and abdomen that can become hemorrhagic, swelling of the spleen, ulceration of the intestines, and constipation or diarrhea. (PARKER, M.D. et al.) Salmonella Typhi lives only in humans. Persons with typhoid fever carry the bacteria in their bloodstream and intestinal tract. In addition, a small number of persons, called carriers, recover from typhoid fever but continue to carry the bacteria. Both ill persons and carriers shed S. Typhi in their feces. (Luby, Steve) Typhoid fever is more common in areas of the world where hand washing is less frequent and water is likely to be contaminated with sewage. Various host defenses are important in resisting intestinal colonization and invasion by Salmonella. Normal gastric acidity is lethal to salmonellae. In healthy individuals, the number of ingested salmonellae is reduced in the stomach, so that fewer...

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Kotton, M.d, Camille. "Typhoid fever." MedlinePlus. 9 June 2005. National Library of Medicine. 2 Dec. 2005 .
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