The human body consists of several organ systems that function in harmony and carry out important functions, such as, digestion, excretion, and respiration. The digestive system that begins at the mouth and ends in the large intestine has several glands that secrete digestive juices. The salivary glands, that remain hidden in the oral cavity and produce saliva has function not only in digestion but also play a role in innate and acquired immunity (Fábián, Hermann, Beck, Fejérdy, & Fábián, 2012). It is interesting to note the role played by saliva in enhancing the immune system whereas it primarily appears to be associated with the just the digestive system. Another part of the alimentary canal, the small intestine apart from the role of digestion has the Paneth cells, modified epithelial cells that play a role in the host microbe interactions and have protective function against enteric pathogens (Bevins & Salzman, 2011).
There are three pairs of salivary glands in the oral cavity- the parotid glands, the sub-mandibular glands and the sublingual glands (Fábián, Hermann, Beck, Fejérdy, & Fábián, 2012). Though the main function of saliva appears to be lubrication of the oral cavity, there are several proteins in saliva that function as defence molecules. Saliva contains lysozyme that is part of the innate immune system and kills bacteria that may enter through the oral cavity. Salivary amylase, mucins, statherin, cystatins, and several cationic peptides are other proteins in saliva that form part of the innate immune system. The salivary chaperokine HSP70/HSPA plays a role in innate and acquired immunity. Immunoglobulins in saliva add to the body's defence armour. Higher levels of salivary immunoglobulin A decrease the adherence of bacteria associated with dental plaques and prevent tooth decay.
In patients suffering from head and neck cancers the treatment affects the secretion of saliva to a considerable degree. Other than lubrication of the oral cavity the secretion of saliva facilitates several functions such as swallowing, sense of taste, maintenance of dental health and talking. The digestion of starch begins in the oral cavity while chewing due to the presence of amylase in saliva (Kagami, Wang, & Hai, 2008). All these functions are affected when salivary gland do not function optimally and xerostomia or dry mouth results (Holmberg & Hoffman, 2014). Hyposalivation results in poor dental health.
Consumption of sports drinks by athletes that have a pH of 4 or less can reduce the pH of saliva whereas the intake of mineral water restores the pH of saliva. Salivary flow rate and the salivary buffering capacity increases on drinking water rather than sugar containing sports drinks by athletes engaged in endurance sports. In case of dehydration, there is greater likelihood of dry mouth (Tanabe, Takahashi, Shimoyama, Toyoshima, & Ueno, 2013).
Salivary glands and saliva are known in vertebrates but not all organisms have salivary amylase as a constituent of saliva. The snake venom is produced by modified salivary glands. Insects have modified salivary glands that they use to produce silk or glue.
The small intestine is largely engaged in the carrying out the functions of digestion and absorption, due to the large surface area facilitated by the presence of microvilli. Some cells of the intestinal epithelium are highly specialized to perform functions other than digestion and absorption and are called the Paneth cells (Clevers & Bevins, 2013). These cells are known to secrete antimicrobial proteins and peptides that play a key role in host-microbe interactions. The peptide secretions of the Paneth cells help in the maintenance of a homeostatic balance with the microbes that colonize the small intestine and confer immunity against the enteric pathogens through innate immune mechanisms (Salzman, Underwood, & Bevins, 2007). The Paneth cells were named after the scientist Josef Paneth who identified these as granule - rich epithelial, pyramid shaped cells stained by eosin and phloxin-tartrazine that were located at the base of the 'crypts of Lisberkuhn'. These cells differentiate from intestinal stem cells that are equipped with a leucine -rich receptor that is G-protein coupled.
Patients with chronic inflammatory bowel disease are known to have Paneth cells that have become dysfunctional and no longer play a role in conferring innate immunity. Paneth cells in mice are known to differentiate after birth while the intestinal crypts develop. In marsupials also evidence of the presence of Paneth cells is available. However, all mammals do have equivalent of the Paneth cells (Bevins & Salzman, 2011). In a study on rats an effort was made to study the influence of a high calorie diet on the number of Paneth cells. The number of Paneth cells in duodenum sections was reduced by 50% after feeding the rats with a high calorie diet. High carbohydrate but non high fat diets showed a similar response in the jejunum sections (Becerril, Castillo-Robles, González-Hernández, & Villanueva, 2005).
In conclusion, it is important emphasize that a single part of the human anatomy may have multiple functions. Thus saliva not only keeps the oral cavity moist, it has an important role in the immune system also. The presence of lysozyme in the saliva kills bacteria and the several proteins in the saliva aid the innate and the adaptive immune response. Since the mouth can be an important point of entry for pathogens, it is important to check their presence before any harm can occur due to infection. The lack of proper secretion of saliva due to disease or treatment can expose the teeth to the risk of infection. Dehydration due to extensive physical labour can alter the pH of saliva and cause symptoms of dry mouth. So constant replenishment of water is necessary for maintenance of oral health. The small intestine largely plays a role in digestion and absorption but for the presence of highly specialized cells in the intestinal epithelium- the Paneth cells that were first described by Josef Paneth. These cells are known to produce antimicrobial peptides and proteins that maintain intestinal homeostasis by killing enteric pathogens.
References
Becerril, A., Castillo-Robles, G., González-Hernández, M., & Villanueva, I. (2005). Influence of high-calorie (cafeteria) diets on the population of Paneth cells in the small intestine of the rat. European Journal of Morphology, 42(4-5):201-7.
Bevins, C., & Salzman, N. (2011). Paneth cells, antimicrobial peptides and maintenance of intestinal homeostasis. Nature Reviews Microbiology , 9:356-368 .
Clevers, H., & Bevins, C. (2013). Paneth cells: maestros of the small intestinal crypts. Annual Review of Physiology, 75:289-311.
Fábián, T., Hermann, P., Beck, A., Fejérdy, P., & Fábián, G. (2012). Salivary Defense Proteins: Their Network and Role in Innate and Acquired Oral Immunity. International Journal of Molecular Sciences, 13(4): 4295–4320.
Holmberg, K., & Hoffman, M. (2014). Anatomy, biogenesis, and regeneration of salivary glands. Monographs in Oral Science, 24: 1–13.
Kagami, H., Wang, S., & Hai, B. (2008). Restoring the function of salivary glands. Oral Disease, 14(1):15-24.
Salzman, N., Underwood, M., & Bevins, C. (2007). Paneth cells, defensins, and the commensal microbiota: a hypothesis on intimate interplay at the intestinal mucosa. Seminars in Immunology, 19(2):70-83.
Tanabe, M., Takahashi, T., Shimoyama, K., Toyoshima, Y., & Ueno, T. (2013). Effects of rehydration and food consumption on salivary flow, pH and buffering capacity in young adult volunteers during ergometer exercise. Journal of the International Society of Sports Nutrition, 10:49.
