7 Barriers

other systems act to protect body openings such as the lungs, intestines and the genitourinary tract. In the lungs, coughing and sneezing mechanically eject pathogens and other irritants from the respiratory tract. The ﬂushing action of tears and urine also mechanically expels pathogens, while mucus secreted by the respiratory and
gastrointestinal tract serves to trap and entangle microorganisms.

Skin is a barrier that cannot normally be penetrated by viruses or bacteria, but even tiny abrasions may allow their passage. The mucous membranes lining the digestive, respiratory and genitourinary tracts bar the
entry of potentially harmful microbes. Certain cells of these mucous membranes also produce mucus, a viscous ﬂuid that traps microbes and other particles. In the trachea, for example, ciliated epithelial cells sweep mucus and any entrapped microbes upward, preventing the microbes from entering the lungs. Microbial colonization of the body is also inhibited by the washing action of the mucous secretions, saliva and tears that constantly bathe the surfaces of various exposed epithelia.

Skin immune function
Beyond their physical role in inhibiting microbe entry, secretions of the skin and mucous membranes provide an environment that is often hostile to microbes. In humans, secretions from sebaceous (oil) glands and sweat glands give the skin a pH ranging from 3 to 5, which is acidic enough to prevent colonization by many microbes. Similarly, microbes in food or water and those in swallowed mucus must contend with the acidic environment of the stomach, which destroys most pathogens before they can enter the intestines. But some pathogens, such as the hepatitis A virus, can survive gastric acidity and successfully enter the body via the digestive tract.
Secretions from the skin and mucous membranes also contain anti—microbial proteins. One such protein is lysozyme, an enzyme that digests the cell walls of many bacteria. Present in saliva, tears and mucous secretions, lysozyme can destroy susceptible bacteria as they enter the upper respiratory tract or the openings around the eyes.

Chemical barriers
infection. The skin destroys most pathogens before they can enter the intestines. But some pathogens, such as the hepatitis A virus, can survive gastric acidity and successfully enter the body via the digestive tract.
Secretions from the skin and mucous membranes also contain anti—microbial proteins. One such protein is lysozyme, an enzyme that digests the cell walls of many bacteria. Present in saliva, tears and mucous
secretions, lysozyme can destroy susceptible bacteria as they enter the upper respiratory tract or the openings around the eyes. Lysozyme molecules have a complex three-dimensional structure. Lysozyme is an anti-bacterial enzyme widely distributed in human body ﬂuids, including tears and saliva. It is also found in egg white and plant latex. Lysozyme disrupts the polysaccharide components of bacterial cell walls, leaving them susceptible to destruction. Researchers are establishing which parts of the lysozyme molecule are effective against bacteria.

Chemical barriers also protect against infection. The skin and respiratory tract secrete antimicrobial peptides such as the β—defensins. Enzymes such as lysozyme and phospholipase A in saliva, tears and breast milk are also anti bacterials. In the stomach, gastric acid and proteases serve as powerful chemical defenses against ingested pathogens.

Rich intestinal ﬂora Within the genito urinary and gastrointestinal tracts, commensal ﬂora serve as biological
barriers by competing with pathogenic bacteria for food and space and, in some cases, by changing the conditions in their environment, such as pH or available iron.

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