Mucosa, host immunity

Once Salmonella invade the intestinal barrier, the bacteria encounter host immune cells such as neutrophils, lymphocytes and macrophages, which engulf the bacteria (Fields et al., 1986 McCormick et al., 1993) (Fig. 5.1). Salmonella serotypes capable of causing gastroenteritis are killed by host macrophages, and effectively cleared by the immune system. Invasion of M cells and intestinal epithelial cells induces a pro-inflammatory response in the intestinal mucosa (Fig. 5.1a). A combination of inflammatory cytokine production, neutrophil infiltration and fluid and electrolyte secretion by the epithelium results in the diarrhea associated with gastrointestinal enteritis (Fig. 5.1a). 

Infection by H. pylori is detected by serological markers produced by host immune responses (e.g., antibodies to antigens of H. pylori) and a breath test. The latter, known as the urea breath test, consists of oral administrations of radioactively labeled urea. This is metabolized to labeled CO2 and ammonia by the urease of H. pylori present in the gastric mucosa. The presence of labeled CO2 measured in the exhaled air confirms infection. 

Ramos HC, Rum bo M, Sirard JC. Bacterial flagellins mediators of pathogenicity and host immune responses in mucosa. Trends Microbiol. 2004 12 509-17. doi 10.1016/j.tim.2004.09.002. 

A healthy host exists in a state of equilibrium with the gut microbiota and in this way this microbiota contributes to a balanced host immunity, to homeostasis at the intestinal mucosa and to metabolism, while the host provides it with a constant and convenient habitat, and at the same time keeps microbial numbers in check. A disorder in the host—microbiota equilibrium may hapjren where a pathogen enters the host intestine and interacts with both the host and the microbiota. If both these two components (host and indigenous microbiota) are resilient, the balance is restored with a consequent recovery of the intestinal mucosa. A disruption of the intestinal homeostasis is the result of a disturbance of the host—microbiota balance that in extreme cases may lead to the death of the host. If the pathogen acquires a niche for itself among the indigenous bacteria of the gut, it will eventually create a new equilibrium between the host and the microbiota with consequent chronic presence of the pathogen. ... 

Inununization has been relied on in the induction of humoral inununity by parenteral administration of vaccines. Antibodies from this do not necessarily reach the mucosal surfaces of the GI tract where most infectious agents encoxmter the host first. Secretory IgA (sIgA) is the dominant antibody isotype present at mucosal sites. Since sIgA inhibits the attachment of bacteria and viruses to mucosa and neutralizes toxins, mucosal immunity can provide the first line of immunological defense. However, induction of immunity at mucosal surfaces needs administration of vaccines directly to the mucosal site. 

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