General Defense Mechanisms Associated With The Digestive System

Mary Gorton

Microbiology Research paper

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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

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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

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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 or no organisms enter the intestine. Normal small intestinal motility also protects the bowel by sweeping ingested salmonellae through quickly. The normal intestinal microflora protects against salmonellae, probably through anaerobes, which liberate short-chain fatty acids that are thought to be toxic to

Mary Gorton

Microbiology Research paper

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salmonellae. Alteration of the anaerobic intestinal flora by antibiotics renders the host more susceptible to salmonellosis. Secretory or mucosal antibodies also protect the intestine against salmonellae. (Giannella)

There now is an oral typhoid vaccine and a new single dose injectable vaccine that produces fewer side effects than the older two dose injectable vaccine. Both vaccines are equally effective and offer 65-75% protection against the disease.

Three commonly prescribed antibiotics are ampicillin, trimethoprim-sulfamethoxazole, and ciprofloxacin. (Luby, Steve)

Even if the patient recovers from the infection, it may have far reaching effects, such as the formation of attachments of the damaged areas of the intestine to the abdominal wall, or the development of a chronic infection that leads to the patient becoming a carrier.

Without therapy, the illness may last for 3 to 4 weeks and death rates range

between 12% and 30%. (PARKER, M.D. et al.)

Hepatitis C virus

Hepatitis C virus (HCV) is an enveloped single-stranded RNA virus, which appears to be distantly related (possibly in its evolution) to flaviviruses, although arthropod vectors do not transmit hepatitis C. Several genotypes have been identified. (Viral Hepatitis C)

Mary Gorton

Microbiology Research paper

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The hepatitis C virus attacks special cells found in the liver called hepatocytes and uses them as a host to reproduce itself.

Roughly 80% of people do not have any signs or symptoms, if they do they

appear