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Mucosal Immune System
Hanaway explains that the intestine is the primary immune organ in the body, containing nearly 70% of the immune cells - more than 106 lymphocytes/g of tissue52. The gut-associated lymphoid tissue, also known as GALT, represents the largest mass of immuno-competent cells within the human body.
He goes on to explain how the interaction between host and bacteria at the mucosal interface plays an important part in the development of a competent immune system:
For instance, commensal* flora are intimately involved in the development of oral tolerance, part of the body's acceptance of something as self53. The ability to recognize food particles and commensal bacteria is critical for educating the adaptive immune system properly.
He tells us that Crohns Disease and Ulcerative Colitis - both examples of Inflammatory Bowel Disease (IBD) are important examples we can use to understand how to evaluate imbalance within the mucosal immune system.
Three factors are required for abnormal physiology to evolve into inflammatory bowel disease:
Hanaway notes that alterations in mucosal integrity that do not include an abnormal immune response will also tend to cause dysfunction within the system, though in a much more subtle manner. Luminal contents that can have a stimulatory effect on the immune system include:
Translocation of bacteria can stimulate a physiologically normal inflammatory response when function is healthy and balanced54.
When a significant number of pathogenic bacteria translocate across the epithelial lining, there is an overt inflammatory response from the gut-associated lymphoid tissue. The upregulation of inflammatory cytokines provides the body with a rapid response to invasion. Multiple mechanisms of cross-talk between the bacteria and epithelia that differentiate between the process of recognition and oral tolerance vs effectively responding to pathogenic bacteria have been described55.
One of the factors noted in the degree of immune response is the adhesion capacity of antigens to epithelial cells56. Strong adhesion of antigens to the epithelial cells is seen with increased immune response.
Now it is seen that Crohns Disease and Ulcerative Colitis -IBD - patients are not tolerant of their own gut flora 57 but, interestingly, the administration of fecal flora derived from healthy controls has been shown to be effective58. A continuum of symptoms from Irritable Bowel Syndrome (IBS) to Crohns and Ulcerative colitis (IBD) has been proposed59 that includes alterations in:
He says that epidemiologic evidence seems to bear out that some of the rapid increase in Crohns Disease and Ulcerative Colitis (IBD) may be due to large numbers of IBS patients, and the continuously increased risk of Crohns Disease and Ulcerative Colitis (IBD) detection in Irritable Boewl Syndrome (IBS) patients favors a true association between the two60.
Hanaway tell s us that probiotics have been shown to have a therapeutic role in the case of food allergies. Bacteria produce a number of enzymes and products to assist with the metabolism of food. For example, Lactobacillus rhamnosus GG was able to hydrolyze casein and reduce the production of IL-4 in atopic infants with cow's milk allergy61.
Probiotics modify the structure of potentially harmful antigens and lower their potential for harm. Current studies are now underway to evaluate the feasibility of creating a group of probiotic bacteria with the capacity to break down gluten in such a manner that people diagnosed with celiac disease will still be able to eat wheat products62.
Commensal* bacteria have effects that extend across a range of immunologic imbalance. He stresses the importance of commensal flora on stabilizing gut flora, promoting the integrity of the intestinal barrier, supporting host resistance to pathogenic bacteria, and modulating immune response. All of these qualities extend to the therapeutic use of probiotics.
The anti-inflammatory and immunomodulatory effects of probiotics provide the basis for therapeutic intervention. Beneficial effects have been demonstrated in Crohns Disease and Ulcerative Colitis (IBD)63, Irritable Bowel Syndrome (IBS),64 atopic illness,65 and food allergies66.
*Commensal: Living in a relationship in which one organism derives food or other benefits from another organism without hurting or helping it. Commensal bacteria are part of the normal flora in the gut.