Principles of Virology, Volume 2. S. Jane Flint

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Principles of Virology, Volume 2 - S. Jane Flint


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membrane. Beyond this extra cellular matrix are tissue fluids, from which particles may be taken into lymphatic capillaries and reach the blood. Local macrophages patrol the tissue fluids in search of foreign particles. (Right) Viruses that reproduce at different locations within the respiratory tract, noted with the associated clinical syndromes. SARS, severe acute respiratory syndrome; MERS, Middle East respiratory syndrome. Adapted from Mims CA et al. 1995. Mims’ Pathogenesis of Infectious Disease (Academic Press, Orlando, FL).

      DISCUSSION

       In praise of mucus

      When you have a cold or sinus infection, it can be disconcerting to take a peek in your tissue after you have blown your nose. However, the thick, colorful mucus that accompanies many such infections actually serves an important purpose. Mucus-producing cells line the mouth, nose, sinuses, throat, lungs, vagina, and entire gastrointestinal tract. In addition to its lubricant function, mucus acts as a protective blanket over these surfaces, preventing the tissue underneath from dehydrating. Mucus also acts as a pathogen flypaper, trapping viruses and bacteria. More than being just a sticky goo, mucus contains antibodies, enzymes that destroy the invaders it traps, and a variety of immune cells poised to respond to pathogens that attach to it.

      It is a common misconception that yellow or green mucus is directly due to the presence of bacteria or viruses. When an individual ac quires a respiratory tract infection, neutrophils, a key element of the host innate response, rush to the infected site. These cells contain an enzyme, myeloperoxidase, that is critical for the ability of neutrophils to eliminate pathogens, as individuals with a genetic loss of this enzyme are immunocompromised, especially for respiratory tract infections. Myeloperoxidase is stored in azurophilic granules prior to release; these granules are naturally green or tan. Thus, when neutrophils are present in large numbers, the mucus appears green. One may indeed assume that discolored mucus is a sign of infection, as recruitment of neutrophils often accompanies infection.

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      One final thought that you may wish you did not know: while it is not a very socially acceptable practice, eating one’s own nasal secretions (mucophagy), a habit of many young children, may have some evolutionary benefit. Some have argued that mucophagy pro vides benefits to the immune system, especially the underdeveloped host responses of children. As noted above, mucus destroys most of the pathogens that it tethers, so nasal secretions themselves are unlikely to be laden with infectious virus particles. Rather, introducing these crippled microorganisms into the gut, where antigen-presenting cells are abundant, may be a form of “low-tech” vaccination or immune memory booster.

       Bellows A. 2009. A booger a day keeps the doctor away, p 28–30. In Alien Hand Syndrome and Other Too-Weird-Not-To-Be-True Stories. Workman Publishing, New York, NY.

      The alimentary tract is another major site of viral invasion and dissemination. Eating, drinking, kissing, and sexual contact routinely place viruses in the gut. Virus particles that infect by the intestinal route must, at a minimum, be resistant to extremes of pH, proteases, and bile detergents. Many enveloped viruses do not initiate infection in the alimentary tract, because viral envelopes are susceptible to dissociation by detergents, such as bile salts.

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